No one is going to replace an emplyee who hasn't even given formal notice with a specific date and rarely will a company budget allow them to pay two people to do the same job. If they are going to move your tasks to someone in house, then usually they will do that in the amount of time after your official notice. Give your minimum contractually required notice period or teh standard two weeks that is the norm in the US. Replacing you is actually none of your business or concern.

Are you okay with being unemployed? If they truly understand you are leaving they might fire you on the spot. Look for a new job silently.

There is nobody in house. I'm the only employee, hence why I gave such a long notice period

I'm fine with being unemployed. Worst comes to worst I could just get rehired at old jobs I've had. But I can also freelance and have a year's worth of savings built up, with a highly in demand skill-set.

I think @HLGEM makes an important point. Either hiring a replacement or considering outsourcing would take resources that they probably don't want to spend until they are sure you are going. Either process would become a mess if you don't actually quit on schedule. The only way they can be sure you are going is for you to give them written notice with a definite ending date.

You fulfilled any commitment you have to them already. Not your responsibility to make sure they take you seriously and work on a replacement, that is on them. If they want your help training, they need to do it before you leave.

How do I convince my employer to take this seriously? Why do you care? Seriously, why does it matter even a little to you?

You're awfully concerned with the well-being of an employer who seems to be exploiting you to the max, where they'll get what they can out of you until you stick a gun in your mouth-- and you keep coming back to them. This is a toxic relationship. Nobody takes a pushover seriously. Unless you own part of this company, stop concerning yourself with its longevity and start worrying about your own.

if they are so small / poorly run that all of their functional eggs are in your basket, you either need to be a co-owner of the shop (and thus get compensated to justify this level or anxiety) or walk away soon

Which country are you in? and what does your contract say about giving notice, and notice period? In many countries, unless you gave formal notice in writing (which you didn't), it may not count. Muttering about it, threatening to do it, giving repeated 'ultimatums', do not amount to giving notice. Get off the internet already and type up your resignation letter. That should take all of 10 minutes.

I'm in the US. I could technically quit this second with no repercussions.

@RichardU: changing the title to 'this' doesn't communicate anything. 'verbal-only notice of resignation' does.

@BrandonPodgorski: a customary notice period in the US is two weeks, where not otherwise specified (but there's no legal impediment to you walking out the door right now, if you don't care about the reference). Answer us this: what is preventing you quitting (in writing) right now? Don't reference 'the boss' or 'the company' or 'finding my replacement' in your answer. What about you is preventing you quitting (in writing) right now? Some employers are simply abusive, greedy or self-destructive. Stop trying to change them! (Or, maybe you're more replaceable than you think)

@BrandonPodgorski" oh, and your written resignation should say "My last day of work will be <Month> <Day>, <Year>", as @TheWanderingDevManager said. It's really that simple. How and whether they react to it is honestly and absolutely not your issue. You can't change them and you can't fix broken companies. You've got to decouple your identity from this job. Get outta there, go sit on some beach, have a beer and rethink your life priorities. But the crucial first step is your written resignation.

@mobileink No need to use that condescending language as in your last sentence. To the OP: do you have a particular reason to trying to be "superfair"? Is it a friend or a relative? It is never a good idea to do business with friends without clear contracts if that were so. But otherwise: if you are so important to the business and you treat the business as if it were your own, what keeps you from getting equity (unless it is in bad shape, and then you shouldn't stay anyway)? In short, what is your rationale of stacking the balance of interests against yourself? Clarifying this is the key.

At the end of the day, I don't want to have equity in this company, as the owner is quite terrible. I might start my own competing business after I leave though.

>"after giving a few … ultimatums about 6 months ago, nothing has changed" — you are doing it wrong! ultimatums don't work that way, you must enforce a change specified there. otherwise they won't ever take you seriously.

Well some took several months (like finding projects using different technologies) so I expected some lag time.

"I might start my own competing business after I leave though." and have them sue you into the ground if there's a non-competition clause in your contract? Might want to check that.

A lot of this commentary is repeating stuff that is in actual questions and answers over the past couple of years, including workplace.stackexchange.com/questions/80601 , workplace.stackexchange.com/questions/62069 , workplace.stackexchange.com/questions/57086 , and workplace.stackexchange.com/questions/82077 .

One word, resign. They're clearly not going to make changes to reverse your decision to lead. Just hand in your notice.