No problem, you can extend the period as soon as the end is near :)
You can always set an extension up to 1 month into future. Sp, if your initial 3 months period ends in 28th March, on the Friday before (nobody really plans to work on Sunday, do you), 26th March you can set the end of the survey administration period to 26th April.
You can repeat that every month, up to 2 years overall survey administration period.
Why a 3 month limitation, then? Because we had lots of survey that were set to run for 10 year, in the beginning. And inactive survey decrease the overall server performance.
Is this something that required a paid license?
If you have a paid licence, than you could set the period freely within the licence period. So if you bought 3 licences for 12 months, each (just theoretically), you could set a 36 month survex administration period from the first day on.