I want to agree with everyone who mentioned network bandwidth. We recently did a push upgrade to approximately 8,000 clients over a 4 night period from 11.x to 12.1.x. I had set up schedules so the push would only happen from 6pm to 5am and it would distribute the upgrades over 4 days. Every day I went in and modified the schedule so it would be one less day. On the last day I modified the schedule to say 0 days - went from 2 days to 0 days underneath the assumption that since this was the last day 0 would be fine. There was still approximately 1,600 clients that still needed the upgrade. As soon as I clicked ok after modifying the schedule to 0 days the SEPM's immediately began pushing out the installation package to all 1,600 clients. The 0 days overrode my timing schedule of 6pm to 5am - basically 0 days means emergency push. I was watching the network traffic and saw the SEPM's hit 100% utilization on their gig links so I immediately backed out the 0 and put in a 2. But about 600 clients were still downloading the 265MB package - brought our network to a halt for 11.5 minutes due to the flood of traffic going out from the SEPM's. Just another learning opportunity....
In responce to your question. I would recommend that you can upgrade your clients during the day or night between a 3 hour period. Then have the reboot scheduled for later that night. That should result in a single SEPM pushing out about 84GB during that 3 hour period - again you have to know the capacity of your network setup from the SEPM to the network core. Hope this helps.