I did this yesterday and it took just over 40 minutes or so, going from T5B directly. Biggest delay was waiting for a bus to stop that wasn't full (third time worked). There is fast track available through the T3 connections security that helped too. Just don't forget liquids in your bag! Myself (and the three in front of me) were all guilty.

Took much longer going back the other way after getting my LHR-HEL-OSL changed to a direct flight since a late arrival from AUS meant I missed my connection and BA only rebooked me as far as HEL on the last flight of the day. Ended up waiting for 4 bus loads in front on the trip back to T5