Normal issue was 40 rounds. If additional rounds were available soldiers would stuff them in haversacks, pants pockets, etc. Supplies of additional rounds would have been in the general area of the regiment. IF the regiment was on the move, rounds could also be obtained from the dead and dying on either side.
"Standard" sustained rate of fire would be about 3 rounds a minute. IF firing at this rate figure a bit under 15 minutes of sustained fire. As firing continued, rate of fire would fall under 3 rnds/minute due to fouling of the barrel, as it was harder and harder to ram the round home.
While a specific battle might be hours long, there would have been a variety of phases within the battle. Normally regiments did not constently fire at each other for two and a half hours, unless they were in trenches like Petersburg. One regiment or the other would break or 'retire.'
There are maneavours in which regiments swap places on the firing line, to permit resupply of the regiment moving out of the line.
Hope this helps
JIM T