What would be the ideal amount of notice that tech companies should be required to give their employees before a layoff?

When you use the word "ideal" in this question, it changes the impact. "Ideal" would be enough time for displaced employees to find other work. If you replaced "Ideal" with "Realistic" I bet the answers would change drastically.

Other: As per agreed in contract

Oh Brother...  How would you approach the challenge you have if you acted as if you cared about each and every person there?    This provocative question Is not about protecting people from discomfort or putting the survival of the company at risk.   Maybe it's to be transparent with what is going on and offering employees a chance to design solutions together such as trading furlow days, or willingly reducing their own pay.    A friend of mine and I believe that offering the option of a startup challenge to employees as an investment into new ventures (which they are going to start anyway and cost a lot more for a tech firm to purchase later anyway.) 

i think this depends on a couple of main factors: on the country where this apply and the seniority / role impacted. i can be aligned on a 90 days on average, but as seniority and role is high a careful and ad hoc management, communications, of this choice should be considered. 

It depends on where they are legally HQd and where they employ people. There are different legal requirements in place by country.
Should they go above the local legal requirements? - I don't think so. Being employed in tech is no different than being employed elsewhere and no other industry offers any privilege here.
I agree with others who noted before me that employees are usually well aware or aren't surprised by these layoffs.
I think the problem has always been with the "how" during the handling of these layoffs.
Nobody wants to be treated like a robot, so if they treat them like human beings and within the legal requirements of the country, they should be fine.
I'd rather go in the other direction with questions though and ask this:
Why there is lack of strategic consideration and proper operational / execution follow up at hiring, when they know they have problems? Why hire when you'd let go within 1 months announcing mass layoffs? etc....but most importantly: why not hire as you truly need and not based on an industry hype?
I could ask many further questions here and none of them would go towards the layoff side, but rather how to manage workforce in a way that you can avoid mass layoffs....as there are many ways to not get there.