Licenses for SQL

We are Shiping systems with SQL7.0 unfortunatly we are running out of licenses, we checked out the Microsoft web site to find out if we could use SQL 2000 licences.

The answer was yes,by taking advantage of the SQL Server 2000 ?downgrade rights.?The downgrade rights enable customers to purchase SQL Server 2000 Server/CAL or Processor licenses, but
install and use the previous version for a reasonable period of time.

The Question :-
What is a “Reasonable” period of time ?
6 months, 1 year, I don’t know, can you help

regards
Jim