There is no easy way to work out how long a TFS in place upgrade will take, there are just too many factors to make any calculation reasonable
- Start and end TFS version
- Quality/Speed of hardware
- Volume of source code
- Volume of work items
- Volume of work item attachments
- The list goes on….
The best option I have found to a graph various upgrades I have done and try to make an estimate based in the shape of the curve. I did this for 2010 > 2013 upgrades, and now I think I have enough data from upgrades of sizable TFS instances to do the same for 2013 to 2015.
Note: I extracted this data from the TFS logs using the script in this blog post it is also in my git repo
So as a rule of thumb, the upgrade process will pause around step 100 (the exact number varies depending on your starting 2013.x release), time this pause, and expect the upgrade to complete in about 10x this period.
It is not 100% accurate, but close enough so you know how long to go for a coffee/meal/pub or bed for the night