What is the effect on battery life of enabling glonass support on garmin edge 25?
You're only going to experience roughly a 10%, (maybe 20% max) loss of battery life. Unless you're planning on frequent century or all-day rides, the difference is relatively negligible. The average use case, a commute to and from work, or the average 4 hour group ride, won't see any difference because the battery life will go from about 8 hours on a full charge to 7 hours.

If you do plan to take it on century rides or night rides, you might want to upgrade to a medium tier unit, 510/520/etc, as the medium sized units have nearly twice the battery life. Using the GPS+GLONASS mode doesn't have a significant effect on battery life at all in this case because we're looking at about 16 hours of battery life in GPS-only mode vs 15 hours of battery life with GLONASS. Unless you're living on the edge (no pun intended), the GLONASS mode isn't going to make it or break it.