I had the same question a couple of days ago. The initial visa is valid for 33 months from the date of issue. Before moving on to the next stage of the 5-year the process for residency the system requires that the person be settled in the UK for an initial probationary period of 2.5 years/30 months, hence the reason why the initial approval is for 33 months to give a small grace period. At the end of the 2.5 year probationary period the visa can be extended to cover the remaining 2.5 years, giving a total of 5 years. Therefore in order that the initial visa does not have to be extended (each time at cost to you) twice instead of once in order to complete the total 5 year period it is advisable, but not mandatory, that she makes her first entry into the UK within 3 months of the date the visa was originally issued.
You can see from my thread that my situation is very similar to the one you find yourself in, albeit for slightly different reasons.
Also don't get confused with "first entry into the UK" and leaving the UK for vacations or any other reason, also a subject touched on in my thread.
http://www.immigrationboards.com/immigr ... 76191.html