when can i cut back a heather bush and how much can i cut it back

The best time to trim a heather is immediately after flowering. Assuming it is a winter flowering heather you're a bit late to do it this year but you may get away with it and still get flowers next winter if you do it now but you are risking cutting off the stems that will flower next flowering season.

The most important thing to remember is never cut back into stems with no green leaves on them as they will not regrow. Otherwise a light trim taking off all the old dead flowers immediately after flowering every year or two will stop them getting leggy.

If your heathers are leggy and have bare leafless stems at the base you have two choices. Dig them up and start again or accept they'll stay looking leggy! :^)