So over the last couple of weeks I have been slowly adding to my "garage", one or two cars a day, I got to the point where there was not much more room in the parking garage, I was trying to keep all the vehicles on the same floor you exit the lift from V's apartment. So I also added the 4 bikes as you can squeeze them in anywhere.
Over this time I travelled all over the map, waiting for different periods up to 24 hours, then travelling back and at no time did any of the vehicles de-spawn.
With the floor full I had managed to accumulate 26 vehicles on that one floor, count them on the mini-map
Also shows on the in game map, here just under the player icon
Personally I hope this feature stays, as I like to have all my cars/bikes in one physical place. There of course is a drawback as some players may not like the "vehicle arrows" at the side of the screen, so hopefully it can be made optional.
Note, I only did this to discover if I could. To me there are only a few vehicles that are "must have" for me.