how long to attenuate at 50°F?

I had a helles fermenting around 48-50 that I was worried would sit to warm when I went on a 10 day trip, since it was about 16 points from done I think (notes are at home).

I put it at 40F for the trip, and it dropped down some while I was gone, but isn't finished. I warmed it to 50F, since I needed to add a pilsner to the freezer, and racked it to a keg. Should I vent the keg each day or anything? Any idea how long I should give it at 50F to work off the last 6 points or so? It tasted good, but I'd like to finish it out before it goes into the 33F freezer.

Yeah, it's got to finish fermenting before you can lager it. I'd just vent "whenever", as often as you need to do it to release co2. After it reaches FG, I'd taste for diacetyl and do a diacetyl rest if needed before even thinking about lagering it. It'll be hard to do a d-rest after it's been racked off the yeast cake.

I think it might take quite a while to finish up, since you lowered it to 40 degrees and racked it off of the yeast cake. Some of the yeast would have went dormant, and are in "sleep" mode. Give the keg a gentle swirl a few times, so that it will encourage the yeast to wake up and be active. Hopefully, you won't have a stuck fermentation.

I didn't taste any diacetyl in my sample when I measured the gravity while transferring (it tasted awesome, even with almost no carb). I'm content to let it sit and mellow a while! I'll check it and release pressure every day or two and hook up co2 and a picnic tap to test a sample's gravity in a week or so.

I had it up to 50F before the 40F while I travelled, then back up to 50F for 2 days when I got home, so I think it had a rough equivalent of a diacetyl rest, since it certainly began bubbling again as the temp rose.

My lagers have typically taken a month to 6 weeks to finish fermenting. When I did my Schwartzsbier ale/lager experiment, the ale was on tap before the lager finished fermenting.