SoundBar,
Ironically, today I just gave my 2 weeks notice. I am leaving a Part 141 University gig for a 121 op. While a few people that do the scheduling will no doubt bitch and moan, I don't feel the middle management ****stains deserve anything more from me. But I digress.
As has been repeated before, it depends. Definitely avoid burning a bridge you may need later if things get rocky. I almost did that to myself once but luckily had networked enough where it didn't matter
I feel that the notice length should be directly proportional to the difficulty in finding your replacement. For example, at a flight school with 50+ CFI's, they have attrition and replacement happening practically every week when hiring is good - it's par for the course. At a smaller 91/135 operator, etc - maybe 3 to 4 weeks - it will be a thorn in their side to find and train a replacement quickly- and yes, they get what they pay for.
My only exception to the 2 week standard minimum occurs with safety issues. I had warned and even issued an ultimatum to owners of a small Part 91 I worked for about the age and quality of equipment. Mx was governed by how fat the checkbook was feeling that month. They scheduled a big trip and I burned em about 4 days before. Cutting my pay was the last straw. Excellent pay and crap equipment: I'll make it work. Crap pay AND crap equipment? SEE YA! I will not whore myself out.
Honestly, who gives a 90 day notice? That would be the longest 90 days of your life, working for an employer who could **** on you for three whole months. A month, maybe even 1.5 but jesus 3? You'd be due for a pay raise right when you left.

Not intending to flame, as I have worked for a few employers that could be civil for 90 days, but I doubt that is the norm.
Good luck to you in your new job, if you are leaving and wherever you are going. Happy New year to you and I hope it can be a prosperous one for you and your family.