How long does it take to recovering ranking in Google?
 

"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 depends on what strategy you are using,
Do on page and off page SEO

Update more unique content and promote it top websites like reddit, stumble upon, blogger...etc, because your website easily get crawl by google and you can get good traffic.

change your strategy and check which part is really effected after update new and informational content on your site.

It totally depend on your work or strategy which you follow while building backlinks for your sites, Do on page and off page SEO