Why six hours? From what I gather, it would be about 60-90 min from Changi to Woodlands train station, KTM from SG to JB is 5 minutes (plus 30 min with clearing immigration). 35 min again for the way back and 60 min MRT down to Gardens by the Bay.

4 hours but obviously there's all this waiting in the middle of each transit so even adding an extra hour on top puts it under 6. Or am I grossly underestimating an aspect of the transit?