Most of the time, advantage is worth about +4 to +5, and it's closer to +5. Disadvantage is similarly substantial.
I thought of some more guidelines:
- Never give advantage/disadvantage for something that the rules already cover. For example, when attacking a large creature, the target's largeness is already factored into their stat block, so no advantage. Now, if the large creature tries to ride a small bicycle, sure, he might get disadvantage because there's nothing in the rules about it but it seems like it should matter.
- If something "seems like it should matter," then usually it should matter both ways, granting advantage sometimes and disadvantage others. Size is a good example.
- Rules should generally apply to NPCs as well as PCs. If "higher ground" is enough to warrant advantage for the PCs, you can bet the enemies are going to utilize it too, and then you will have some VERY sad PCs.
- The normal rules already cover all the normal situations. So the DM only needs to hand out advantage/disadvantage in truly unusual situations. If players devise some tactic that can get them advantage all the time ("trusty battle stools," lol) then it's fair for the DM to say, "Nah, if it's that significant then everybody's already doing it, and it's already baked into your regular stats."