With a sharp bit, cutting a softer wood (like poplar), I'd say maybe 5lbs max. And with a really dull bit cutting maple, I'd still say much less than 25lbs, even if the bit was so dull it was smoking.
40-70 ipm is slower than a lot of hand routing, if you think about it. And again, a lot of times you'll remove more material than 1/8" when hand routing? Have you ever put 25lbs of force on a handheld router. Maybe, but not often.
A general rule of thumb I read a long time ago was to figure 30lbs of force. I like to say 30-50lbs to be on the safe side, but on average, I'd say more like 10-20lbs. Depends on how much the router can handle. With the right tooling, like chipbreakers or roughing bits, you can take deeper, faster cuts without putting more load on the router. They'll cost more, but they'll also let you get more out of your machine.
I really need to get my machine done so I can have some real world numbers to back me up. Just to make sure that using 10HP routers all day isn't skewing my view of reality.
On a somewhat related note, or maybe not related at all, I see a lot of people worried about going too fast and breaking bits. Our router at work uses aluminum based pods, on linear shafts that lock pnuematically. The locks are strong enough for the pods to not move while routing. A few months ago I was doing a test cut with what I thought was an 1/8" x 1/2" long tool, but in reality it was a left handed, 1/2" x 1-1/2" tool. Long enough to reach through the board, and crash into the aluminum base of the pod, while spinning the wrong direction. A right handed tool would have cut into the aluminum an inch or so, then get stuck in there real good and break. But because the left handed bit wasn't cutting, it just pushed the pod along the rail, with about 100 lbs of force, until I could hit the button. The bit had a little aluminum stuck to the tip, but I was able to keep using it for awhile longer, since I don't normally use the tip section of it. So figure a solid carbide router bit good for about 100 lbs of force, if it's not spinning but you're still trying to cut with it.