Change of status from L1 to H1

[acasks]

I am currently on a L1 visa. My H1 was approved in Nov 2006. Can someone tell me how many months I can continue on my L1 before I apply for a change of status to H1. Is there a limit on the no of months I can have my H1 idle while I continue to work on L1. Any inputs on this is appreciated.

I guess excessive delay in applying for a status change could cause problems but I want to know whats a reasonable time frame.

Thanks.