A 510 can last me 4 hours if I'm busy. If I'm sitting at my computer (replying to forum posts for instance) it only lasts about an hour.
Everyone talks about getting bigger batteries and I always think about how long my real cigs used to last (less than 5 minutes) lol!
Keep it simple. She's 55 and won't like "change" If you keep her as close to a real cigarette as possible you'll be better off. Get her an auto battery to start with tobacco flavor (or menthol if that's what she smokes). After she likes it you might be able to upgrade to a manual battery.