How much time do I have to wait before I can sell another car?

48 minutes real time, which equals one day in game.
The timer also continues running when you play singleplayer or go offline.

Although @sdfghjk's (that's a mouthful!) answer is accurate, let me provide some more information:

You can sell a car to Los Santos Customs once per in-game day, which is every 48 minutes. Note that you always need to wait an entire 48 minutes between selling cars, so if you sold one at 9:03pm on one day, you couldn't then sell another car at 1am the next day: you would need to wait until 9:03pm. All servers are synced to the same in-game time, and that time elapses regardless of whether people are logged in (although I can't comment for what happens if the servers go down).

GTA Wiki (not an entirely credible source):

GTA Online's time will correspond to every player in the session so if the time is 22:00 in one player's game, it will be the same for every other player.

Time is live in GTA Online, so leaving the game, and returning 10 minutes later will result in GTA Online's time being progressed by 5 hours. Again, this applies to all players, and all sessions; all sessions will be the same time.

and another wiki page:

Players can only sell a vehicle once per day (in-game) or 48 minutes in real-time.

IGN:

you can only sell one car per in-game day (48 minutes).

I can also confirm from playing the game through Steam (I cannot link to my profile as I didn't purchase the game through Steam, but I added it to my library) that this is accurate. I do my best to sell cars as often as I can, and it's often the first thing I do upon joining and if I forget to track the time (either via ESC or checking my in-game phone) I check the Steam overlay and see how long I've been playing.