(vs S3/4 WS3)
for every wound cause you need to hit (1/2), wound (1/6) and fail the armour save (1/6). 10 x 2 x 6 x 6 = 720
at the other end of the scale, such as white lions, bestigor, etc...
(vs WS4+, S6)
you hit (8/9 with rerolls), wound (1/2) and fail save (1/2). 10 x9/8 x 2 x 2 = 45.
This makes it seem like it's quickly toasted by these GW elite units, but when you realise they're only getting 4 files to attack, even in horde (and I don't think I've seen a white lion horde), they can only bring to bear 12 attacks a turn, so they'd still need four rounds of combat to take it out.
Knights will hit with a similar force, but will have less attacks than an infantry horde, and usually have the added penalty of becoming S3/4 after the first round (or even from the first round if you charge them), but even a 15 man bret pain train can only bring 12 attacks to bear in the first round.
GW monstrous infantry seem to be a more worrying option. A horde of GW minotaurs, who could bring 30+ WS4 S7 attacks to bear would mince it pretty savagely. Fortunately you'll never see such a unit. an Irongut horde on the other hand...
(vs WS3 S6)
hit (1/2), wound (1/2), fail save (1/2) = 10 x 2 x 2 x 2 = 80 attacks. such a unit can put out 27 attacks a round before adding characters into the mix, should kill it in 3 rounds of combat.
So, avoid GW elite troops, especially if they have rerolls to hit, and GW monstrous infantry. But anything else... go for it.
Oh, and I've just remembered, on top of this you cause terror, so you might be lucky to have anyone hitting you on 5s!
Here's a little summary table of the number of attacks different WS and S combos will need. (for units with rerolls to hit, it will take 2/3 the listed amount, for WS4+)
|WS||S4 or less||S5||S6||S7|