How long your piercing takes to close varies. Some people's piercing begin to close almost immediately. Others will stay open for years at a time.
As a body piercing apprentice I have seen people come into our shop after having a tongue ring fall out while they were asleep and need to have the piercing tapered back open it had closed so quickly and tightly. On the other end of the spectrum we had a woman come in because she had taken her tongue piercing out 2 years before and she wanted it re-pierced. We discovered she still had her piercing, it was completely open and just needed a little stretching to fit jewelry back in.
The lesson is: No one knows what your tonge is going to do. The only way to know is to take it out and see.
So you have two options you can hang out at home one day, take your tongue ring out, and try putting it back in every few hours. If it slips back in after 2 hours being out, try 4. If that works, try 6, etc. You can even leave the piercing out overnight if you want to test it. Or if you're not a risk taker or just don't have all day to mess with your tongue ring, then you can buy a retainer.
Retainers can be found at most tattoo shops, many mall stores, and online retailers. There are different styles available as well as varying qualities. To get a good look at your options, visit the tattoo shop or stop by the mall and see. You could also look into getting clear balls for your regular tongue barbell. Since you have to wear the retainer so often, you need to buy one that can stand up to wear and tear or you will be buying a new one every week.
Body piercing apprentice