Visa run to Oman every 31 to 39 days, don't do it before 30 days as you still have a current one, it wont be renewed... They are classed as muliple entry now. Fine on day 40 is 340 AED, then 100 AED per day after that!
For UK passport holder the visa can renewed for an unlimited number of times.
GB, Ireland, USA, NZ, AUS, Cananda, Singapore, Malaysia, Hong Kong, South Korea, Japan, Brunei and Seychelles are the only other countries that can.
No need to fly out on 90 days at all.
GF can look for a job while she is here if she wants to work and would not have to leave to change her visa status.
Unless you bring attention to yourself co-habitation is fine.
Do a google search of visa run tour companies as they have up to date info. Go Tours Dubai are pretty good at keepng up to date. Ex pat woman is good too...