Not sure how to count when we became "full time" generally I count it from the day we replaced our NY driver's license with Texas licenses and registered to vote in Texas. Prior to that we had been traveling in the motorhome for longer and longer times, eventually we stayed away from our home base for 11 months and realized we did not need the upkeep of a large townhouse with all that meant, especially neighbors who were not always friendly.
We moved necessities and collectibles into an apartment that was 1/3 the size of the townhouse and declared that to be "expensive climate controlled storage" This past summer we spent two months there, July and September (August we traveled overseas). 2016 I expect we will be there part of July and part of August, maybe a bit of October too, and more overseas travel.
We consider ourselves full time. The apartment is a summer home! We have moved every couple of weeks or more often, except when visiting our children since 2011. We plan to stay put, sort of, at Jojoba Hills Escapees RV in SoCal for much of this winter. It remains to be seen how well we will do at staying put :)
I guess this is more of how then why. Why is because we enjoy the lifestyle and the people. We have friends across the country, some are RVers, and some are from our life before RV. We do make friends easily and we cling to old friends too.