1) The voltage will depend on the charger.
2) The voltage will depend on the temperature.
The charger will have a float range, not a single point. It may try to keep the battery within this float range with a trickle charge, or with a load-dependant charge, or simply by letting the battery voltage fall, then switching to BOOST until the battery hits the boost return voltage.
The "optimal" voltage is slightly temperature dependant. Some chargers can adjust for the temperature dependance, in which case the float range will be adjusted. And since the battery may be at the top, or the bottom, of the float range when you disconnect it, its voltage may be higher/lower than it would be at a different temperature.
If the charger is charging when you disconnect it, either because it is trickle charging or because it is in the low part of its float range, and there is no load connected, the battery voltage will fall when you disconnect the charge current. But the battery is capacitive, and full of migrating ions, so the voltage continues to hold up for a while after you disconnect a charging current.
And if the battery is hot because you have been using it previously, the open circuit voltage will drift down as the battery cools: it could be at the correct float voltage when you disconnect it, but some time later you could find that it is lower.
(And some time much later, you will certainly find that the battery voltage is lower, because of self-discharge)
Finally, if the charger is not charging, and there is no load, and so there is no current going through the wires ... and the battery is at the specified temperature, and the environment is at the specified temperature ... then disconnecting the wires does not change the battery voltage, and the average cell voltage for cells in good condition should be the specified voltage.
Measure open circuit panel voltage in bright sunlight - ideally will be 17V or more.
And/or count "cells" in panel - usually 36 series cells for a 12V lead acid charger suited panel.
Check current - put DMM etc on 200 mA range or Amps range and measure panel short circuit current into meter. This should be say 100 ma+ for 100 mA panel
Full sun hours to charge ~= 7200 / Ichg mA.
0r 7200/100 =~~~ 72 hours in your case.
Equivalent full sun hours are much less than hours of daylight.
Google
gaisma your_city_name
and read kWh/m^2/day figure from table to see how many SSH (sunshine hour/day) are available at your site. 6 SSh/day is high in Summer.
So you would need about 7200/100/6 = 12 days in mid summer to charge the battery completely.
Best Answer
If you charge at float voltage, not only will it take a long time to take as much charge as it's going to get, it will also not reach 100% charge.
It depends what it's going to be used for.
If as a mains failure backup battery for something like a burglar alarm or safety lighting, then floating it is a sensible thing to do, as it maximises life of the battery under those conditions, and makes for a very simple charging circuit, one voltage regulator and you're done.
If as a camping/caravan battery where you do want to know that you are up to 100% SOC, no, it won't give you that.
A word about lifetime and battery chemistry. Lead acid doesn't like being discharged to below 50% SOC, it shortens its life significantly if you do this repeatedly. It therefore makes a good alarm standby battery, but not such a good camping battery, where you might be tempted to run your peltier fridge up to 100% of the Ah claimed on the battery.
There are better battery types for deep discharge, NimH and LiPo to name the obvious ones. Neither are as cheap as lead acid (if you have a vehicle to transport the weight). If you want a long lifetime from your lead acid battery, then double the Ah you buy initially, so you don't need to fully discharge it.