Starting in February, the sessions are from 4pm - 7pm. On alternate months the time will change to 7.45am - 10am.
I think that means this month's session will be at 7.45am - 10am
this Thursday 10th March.
The service - marking your bicycle with a code in indelible ink and registering it in a police database - normally costs £15, but will be free courtesy of the Met.
Using two good locks ought to prevent your bike ever being stolen, but should the thieves succeed, then registering your bicycle is probably the most important thing you can do to ensure its safe recovery.