As far as I can tell, it is stored in the command column of msdb.dbo.sysjobsteps as an argument passed to master.dbo.xp_sqlmaint. Look for something like:
-DelBkUps 3DAYS
I don't see it anywhere else. Have a look at the following tables:
select * from msdb.dbo.sysjobs
select * from msdb.dbo.sysjobsteps