Hi,
The 3 month interval is pretty much standard. It is unlikely for non-small cell cancer to increase in size in one month, and it seems that the strategy seems to work. The interval would indeed increase in the next year (as ipnotica mentioned) and so on.
At her age, the estimated life expectancy of her peers is around 11 years, so it seems reasonable to give her the same care as a younger patient would have.
Stay positive.
No, I don't think it's too much. It sounds like the doctor is being cautious by monitoring treatment and frequently watching for any changes. After a few scans with good results, she'll likely graduate to having scans every 6 months and possibly even graduate to once a year.