You can choose to redo it now, or to wait and see what it's
like in a few days. I'd pick the latter.

If you decide to do it again, try a different adhesive. I used Ardex Flex
7001 Timber System and it was excellent. It certainly didn't take 48 hours
to set. The tiles were 600x400mm slates. It's been down for over a year and
there hasn't been so much as a crack in the grout (Ardex Flex FL).

http://www.ardex.co.uk/ardexflex_7001.asp
http://www.ardex.co.uk/ardexflex_fl.asp

Christian.