The documentation regarding the Stable flag:
"Stable" -- A router is 'Stable' if it is active, and either its Weighted
MTBF is at least the median for known active routers or its Weighted MTBF
corresponds to at least 7 days. Routers are never called Stable if they are
running a version of Tor known to drop circuits stupidly. (0.1.1.10-alpha
through 0.1.1.16-rc are stupid this way.)
(That's a direct link as of the time of writing. Look for Stable
3.4.2. Assigning flags in a vote)
FYI for in the future: many (but not all) torrc changes only require a reload as opposed to a restart. Changing your ContactInfo is one of those things. That would have prevented the downtime.
Yes, to get the Stable flag you'll need to wait until your Mean Time Between Failures is above the network average or at least 7 days.
Don't worry too much about needing to restart your relay. Keeping your OS and Tor up to date is more important than shooting for as large of an uptime number as possible.
Just sit back, relax, and feel good about yourself because you contribute to the Tor network. Thank you for running a relay!