So I thought as a point of interest I'd outline why the "three and a half hour firefight" simply cannot have been an actual firefight that (1) used the rate of fire advantage of the Spencer for (2) three and a half hours.
Peak ROF of the Spencer was above 10 rounds per minute, and there are 210 minutes in 3 1/2 hours. Assuming that the average rate of fire was ca. 2 rounds per minute for 90% of the time and that the other 10% of the time was burst fire with everyone firing at 10 rounds per minute, then that means the number of rounds fired during the fight by the cavalrymen would be on the order of 600 each.
The total mass of a Spencer cartridge was on the order of 30 grams. Multiply it out and you have every single cavalryman expected to carry 18 kilos of ammunition into combat - which is obviously too much. (If they were firing at full rate for the whole of the 210 minutes then the total comes to a literally staggering 63 kilograms!)
A more reasonable estimate of the amount of ammunition carried by a Spencer unit is one issue cartridge box each, which held 10-13 tubes each of 7 rounds. This would be 70-91 rounds and would weigh about 3 kg; if these were expended over a period of 210 minutes this would indicate an average per cavalryman of one round every two minutes.
This is of course all speculation, because we still don't know what battle the engagement was actually at. Let's see if we can work it out:
A 500 man skirmish line of the 5th New York was attacked by a division of Confederate infantry. The New Yorkers held them off until there ammo ran low. After suffering 80 casualties, they withdrew. The Confederate commanders reported that they had engaged an entire brigade. They had no idea that 500 cavalry men armed with Spencers had stopped them in their tracks. The point being, even a small number of Spencer Repeaters could have dramatic effect on a battlefield.
So this is the 5th New York Cavalry. Because they're armed with Spencer repeaters it's not 1861 or early 1862, and we're told that they suffered 80 casualties (which is probably a rough number rather than an exact one).
Investigation reveals that this seems to be Parker's Store, at the Wilderness, though the total reported casualties for the regiment at the Wilderness were 63 (18 KIA, 21 WIA, 24 captured). However, they're reported as "about 1,000 strong" (and were fighting as part of the 3rd Cavalry division, not alone) and during the battle they had to retreat five miles over the course of five hours.
It's actually hard to tell just how much the Spencer mattered in this engagement; from examination it's Heth's division attacking Wilson's cavalry division and driving them back five miles over the course of the day. They managed to prevent the Confederates taking the Brock Road, but if the units in question represent a pro-rata fraction of the formations involved then:
Wilson's division should be expected to be between 5,000 and 6,000 PFD, as 1/3 of the Union Cavalry Corps.
Heth's division should be expected to be not much more than 7,000 PFD (as about 1/9 of the Army of Northern Virginia - the AoNV contained eight infantry divisions and the cavalry corps).
So ~7,000 infantry attack ~5,500 cavalry and drive them back five miles.
Even before you look at what weapons they're using this is actually pretty good on the part of the
infantry.