Cant answer these questions-
It will take as long as you want to go through them. Lost river is great, but if your chlosterphobic (??) or a large person- your going to have problems, polar caves is cool too, but not as 'exciting' as lost river is, being that you can crawl through the lost river etc.
We can spend an entire 1/2 day at either of them, if you take your time, read the signs, explore the interior etc... other people are gone through quickly.
We go to lots river every summer...never been to flume gorge.
Brandy