Tier1-general, what is timeframe by which you must travel?

Once you get the work permit under Tier 1 - General, what is the timeframe under which you must travel to the UK?
What happens if you dont travel within this timeframe? Will the visa be considered expired?
Is only a travel needed without a minimum stay? For example, can I go to UK, and come back in a months time and go again at a later date?
Is only the main applicant required to do this or dependents have to meet the same condition?

Thanks a ton!

Reading this thread might help!!