There are a lot of variables in play so a perfect answer isn't possible, however, calling 18-24 months BS is almost certainly wrong. 18-24 months sounds like the ideal answer.

Note that you can spend $8k/month or you can spend $2k/month, depending on where and how you want to live while you're founding your startup. The nominal dollar amount could be a huge range.

You could also reduce the dollar amount of savings you need by the revenue in your MVP - if you're making $1k/month on a software startup you'll probably be able to net that against your monthly spend, even if most of that $1k/month is currently going towards fixed costs. The difference between an MVP that generates revenue and one that does not is enormous.

YMMV, but personally, after a startup experience that was tough both emotionally and financially, I got a stable job and have promised not to do that again unless or until I can save up at least $300k on top of retirement savings. Basically, about two years worth of expenses plus some buffer.

It varies for everyone, but I personally think $100k is a good goal. It's enough that you can sustain yourself for quite a while and make investments in the business without worrying too much about the immediate costs.