Poor for stopping spam. It's pretty easy to write a little bot that just logs in to ycombinator on an active basis. Data would be good though.

Sorry, I meant days since signup, not time since last login. I haven't see any data that suggests spammers are using old accounts, but I'm not sure.

Even if spammers aren't using old accounts, if something like that was implemented, spammers would just add

  sleep( n * 86400 )

to their bots. Once those n days were up, there would be yet another wave of spam.

I think this could work. If you make it something like 60 days that's not too harsh on new users, but will slow spammers down. If a spammer does wait 60 days and post an obvious spam link then gets banned they are stuck waiting again.

The only problem here is a spammer that creates a ton of accounts and then uses them all at once after the 60 day threshold.