Transponder Ident Signal Time
 

Hello all,
I'm working on an ATC system which code/callsign pairs the transponder code to the ATC flight data to give us the aircraft callsign on the radar screen. We've encountered some problems with the use of ident and want to work around that by making the radar system software wait to pair after the length of time an ident squawk lasts.
So my question is:
Is there a specified time which the transponder shall broadcast an ident signal for? Is it laid down somewhere?

Thank you.
2lo