How long after PhD Major Revisions sign off
I can't answer the question per se (as I guess no other than the assessment committee can). But I can add my personal anecdote :)
At my University there is a general rule of 2 months assessment time (1st handin). But for me after about 1.5 month I got a mail saying that the Ph.D. School of my university forgot to send my thesis to the assessers.

In general I would suggest that revision should not take more than 2 months.