Somewhere between forever and forever. I have used the same private proxies for posting since like Jan of this year, maybe before that.
It will take about 5 mins of posting for them to be akismet banned. Akismet banning can happen with only a few manual posts. So you just don't worry about the small percentage of blogs that use akismet and you just keep moving for the mass that doesn't use akismet.
For those blogs, it doesn't matter. Whats to block you? You can only be blocked server side by the blog its self/admin. If they are going thru that much trouble to take the extra effort to block your IP, then they can see thru spam and your never going to get a link on their site anyway. So again, don't worry about this ultra small percentage.
Which leaves you with no other way something is going to block the IPs. Google nor the public can see what IP the comment link came from, so again, "IF" your private proxy provider can handle all the connections your sending at it for fast posting on only 1 proxy . Then you could easily buy only 1 private proxy and use it for a year for past posting, no issues. I have recommended this to people that only want to do some light posting.
I would personally probably grab 10 proxies for diversity, but you don't need more then that when fast posting for sure. UNLESS you are sending more simultaneous connections to those proxies then they allow, in which case you would want more proxies and possibly from a different provider (I like to use a few to spread out my simultaneous connections among various servers). Thats another can of worms though, beyond this discussion.