My guild wants to use time based points system. Is there a way to do this with grss?

Little more information?

There is a command in GRSS that will take periodic snapshots: /grss starttimer

Don't know if that's what you're asking for.

i know about the commnd line. Just wondering how to set it up (if possible) so that it will award .25 points for every 15 min in raid as opposed to boss kills.

Doing /grss starttimer will start the timer. Enter 15 into the popup box, and then it'll take a snapshot every 15 minutes. Keep in mind that this will result in a TON of snapshots, but that'll work for your points. You award the points themselves during the upload process.