For good carriers delivery report is instant, Vodafone, AirTel I mean. If someone's phone is off/out of range then SMS delivery report won't come, that's quite normal, but it WILL come as soon as one's phone comes back to active network, again recipients phone has to be on a good network too. Also delivery reports are 3G/2G dependant, if you are on 3G always then delivery reports are late most times (with accurate time stamp though), but on 2G its instantaneous, that's why I always use Intelli3G module.
But if you are on BSNL, Aircel or those kinds, I don't know, I know that BSNL sometimes takes as long as 3 days for the delivery report, and I think the delivery reports self expire in 72 hours anyway, so you might not receive any report in that case at all, sheer class