Assuming that you have a battery switch or similar method of removing all loads from the battery, battery voltage will decrease over the course of 12-24 hours, and then settle out. The time it takes depends on the battery as well as temperature. In very warm conditions, it will settle out faster. Once settled out, the battery voltage should not continue to drop until a very long time. Also, this final resting voltage can be used to judge the state of charge with ~12.65 being 100%, ~12.55V being 80% and ~12.2V being about 60%. These alternators often don't get the battery quite up to 100% like a dockside charger would.
If the battery voltage continuously drops, then that would obviously be more concerning.
