Until they're not.
Generally speaking, the preterm labor guideline is to call if you have six contractions in an hour, but I've seen that many times before and still not had it actually be preterm labor.
More importantly than six in an hour (which averages out to a contraction every 10 minutes) is are they getting STRONGER, LONGER, AND CLOSER TOGETHER? If you are preterm, even just two out of those three would probably be reason to go in and get it checked out.
Stronger: How much of your attention do these contractions command? Do you have to stop what you are doing to breathe/cope? Is it pain or just a noticeable tightening?
Imagine you were at a job interview, negotiating your new salary. Are these contractions intense enough that your new boss would stop in the middle of offering you a lot of money to say, "Hey, are you OK? Do you need to go to the hospital?" just by looking at your face? Braxton Hicks (practice) contractions rarely HURT, you should be able to fake that they aren't there if the situation required it.
Longer: How long is each contraction lasting? Just a few seconds? 15? 30? 60? 90? If they are increasing in length, that is a concern.
Closer together: Contracting every 15 minutes seems disconcerting, but if they just stay at that, or space out more, or the pattern is wonky (like once it will be 12 minutes and then nothing again for 40 minutes) its not a big deal. But if you go from 15, to 12 to 10 to 8, you should go to L&D to get checked out.
Dependng on how close you are to your due date, its possible that medication can be given to stop your labor, or that a shot will be given to help your baby's lungs develop. More likely though, you will be monitored and sent home, not in labor after all. The thing is though, your care-provider would rather have that happen than have you deny to yourself that your contractions are really labor and not go in until it's way too late to do anything about it. Different hospitals have different policies about when they will attempt to stop labor and when they will just let the baby come. If you are dealing with concerning contractions, you should ask your care-provider when the guidelines change for your particular birth place. The six contractions in an hour is a guide for them to be able to stop labor, and if you are past the point where they would choose to do that, many care-providers would say to just come in when your contractions are about 4-5 minutes apart.
Please remember, if you are ever unsure about what to do, call or your doctor or midwife to get medical advice!