With Hobbits, I want at least 50 guys (or 49 guys and Lobelia
) in there at 500pts, even with some allies.
With High Elves, I'd settle for 26 (two full warbands), as its almost the only option with such pricey figures.
Just saying 'it depends' doesn't quite cut it though. A nice way to look at it may be this: how big could you make the army with the most basic of troops and leaders? So for example just Dwarf captains and warriors (stay reasonable, give them a bit of equipment). Then, you can probably get away with sacrificing about a quarter of your force (in terms of models) to upgrading the remainder (better troops, named heroes, banners, etc.). Wouldn't quite say a quarter of the points, as expensive heroes can disrupt the image quite a bit. A bit more can be spent here if you'd otherwise be left with just enough points to get just a warband leader without troops to lead, although you can distribute the included troops as well, depending on the size of the force. Generally speaking: more is better, but you also will require some quality every now and then.
About 1-2 large monsters or heroes (>100pts) will be enough in nearly any force, possibly 3 in 1000pts. Most things are worth their points reasonably well, but combinations have to be optimal. Cheap troops are cheap for a reason, so be sure to take plenty of 'em.
[Disclaimer: just made the 'quarter' rule up, but I think something along those lines could be true for a variety of forces]