My traffic was literally (and depressingly) cut by %75 due to all of the website downtime I had been experiencing using Nameheap as my webhost. My website was in a constant state of error 500's for probably 1-2 weeks. I have since moved to Siteground, which is so much better, and they worked with me on dialing back a few RSS functions to make sure that I did not over-extend on resource use, but the errors are gone for the foreseeable future. However, Google has a very tiny drop of my pages indexed and my traffic is still down by the %75. All of this server move happened within the past week. Does Google come back and re-crawl and at re-index after a period of time? (hopefully)
It can take from 1 week to few months, patient is key, you can make use of cloudflare to avoid any down times and decrease the load of your site.
not necessarily, some may come back, all may come back, rankings could be better, or could be worse, with google you just never know, as google is unpredictable.... Google may not trust that your site is reliable enough to remain online thus not giving back everything you have lost, however, that does not mean give up, just keep trying and working your way forward and prove to google that your site is reliable enough to get your rankings back to where they used to be.... your indexing will ultimately return, however, just because they are reindexed, that does not mean the ranking for those urls will come back the same.
Keep working at your site, adding content, etc. No one can give an exact time frame but hopefully you'll recover at least some of the traffic soon.