[AccessD] Source Code Escrow

Hi Group

I have been asked about Source Code Escrow regarding a system I have
written.  My understanding is that I deposit the source with a third
party 
and should my business fail or it fails to deliver support etc for the
product the licensee can apply to have access/a copy of the source code
to allow support/further development.

Any one any experience here, pitfalls etc. Another question is what sort
of fee (subscription) to charge the licensee - is there a rule of thumb,
standard fees etc

Cheers

Richard