Regardless great fodder for history jokes. Same as on Bewitched.
I dont know why, but Al Lewis merely mentioning having tea with Napoleon makes me guffaw. I guess I'm a softie for history jokes.

sunlight clearly doesn't impact them as it did Dracula in the Universal series
I remember my younger brother and I watching the show way back when, and in the episode where Grandpa had turned into a horse, he pointed out that not only was he out in the daytime, but had a reflection. Monsters were my thing, so I was pretty proud of him.

While their ages are brought up, the show never discusses their lifespans. And unlike the witches and warlocks of
Bewitched
, they never once refer to themselves as
immortals
. So we can only guess based on what we know of their Universal Monster counterparts, upon which the characters are (very) loosely based.
Grandpa and Lily are vampires, so presumably they are immortal but susceptible to being slain (although sunlight clearly doesn't impact them as it did Dracula in the Universal series).
Herman appears to be potentially immortal. There's numerous jokes about him having defective or damaged body parts which needed to be replaced, so presumably as long as there's available spare parts and a mad scientist available, Herman could live forever.
Eddie is a bit harder to guess. In the Universal Monster series, Larry Talbot (the Wolf-Man) is always trying to find ways to either rid himself of the curse or to get killed so he won't go on killing innocent people himself. His status as a werewolf makes him nearly indestructible (even being revived after getting killed by silver weapons) but there's no mention of immortality. Eddie's situation is different in that he appears to have been born a werewolf as opposed to cursed to be one. So who knows? Maybe Eddie is immortal.
Marilyn is a non-monster human, so presumably she'll live a normal human lifespan.

Probably indefinitely, b/c they were like zombies, I think, in other words I think they had already died, and were the living dead now. Both Lily and Grandpa both mentioned dying before.