Service tax on Advance recd towards fees or OPE

Dear Friends,

Kindly clarify  the following points.

1 Advance amount is received by a professional company on account of fees or out of pocket expenses(yet to billed by the company) from a client.Whether Service tax is applicable. If so, how to arrive at the service tax amount and when it should be deposited-(on the date advancee  was received or at the time of final settlement)?

Thanks and regards
Ganesh