Well, that's 100 minutes, which is kind of overboard. I would definitely knock that down if I were you.
Basically, session length is the answer to the following questions:
"How long am I going to stay logged in if I don't select [keep me logged in]?"
"To how many seconds is the 'Who's Online' feature accurate?"
Those really don't need to be any longer than 5, 10, or 15 minutes at most really, unless you have people that absolutely refuse to let phpBB set a cookie, and even then it's not going to be too inconvenient, as the session time renews every time they click on a link (as I explained in the other thread). So, if the session length is 10 minutes, it will log them out after 10 minutes of sitting idle. If they log in, are idle for 2 minutes, then click a link, they've got another 10 minutes before they're timed out.