I always go with a 60 day visa and get the 30 day extension. My return flight is at 89 days pass my initial arrival flight. Been doing it for years without a problem.
Make sure you have a reason why she'll need to return to Thailand. Getting my wife a visa, we showed that she had a house/home in Thailand. Also showed that we could financially afford the vacation/trip.
We just returned from Thailand after 3 months. We do have a home in Nonthaburi and have our own car, so we get away with some of the expenses. We do take a little vacation every month, like 3 - 5 days, depending on the location etc. This year we spent in the range of 125,000 baht for 90 days. We usely budget for around 50,000 bath per month.