Naio,
When charging, battery voltage will be whatever voltage is being applied to the battery by the charging source... be it rising voltage during bulk/cc, or steady voltage during abs/cv, or constantly changing voltage using solar.
If it's resting voltage you're concerned about, it takes several days for a good surface charge to self dissipate. Yes, you can take a wild @zz guess at how long to run a small load to discharge the surface charge, but a guess is still a guess. This is why I prefer to familiarize myself with my bank, while it's out of service, first, then watch how daily cycling affects things, if any. Sudden changes in bank performance are a definite red flag that something may be amiss, or perhaps it's just a response to temperature, or just a matter of decreased performance, due to being out of service for an extended period of time.
Do as Mex and Landyacht do, and get to know what works best for your agm's, using mfg recommendations as general guidelines, and tweak things from there.
You can also do a Capacity test once or twice a year, to see how the bank is holding up; although you will have to rely solely on voltage readings, throughout, not sg readings. And consider rotating batteries that are in a series and/or parallel configuration, after each C-test, so that the draw is not always off the same battery.
As for using 12v solar panels with a 24v bank... you'll likely need at least three 12v panels in series, to generate sufficient voltage to maintain a charge, if using an mppt controller. Using pwm, you may get away with two 12v panels in series, but it wouldn't take much for the combined panel voltage to drop below the ~ 29v required to sustain a charge.
