I`m reading the exploration challenge and it doesn`t say how many rolls the players can do. The thing is if they can roll until completition of the challenge, there is no point on plan if they fail, they`ll eventually succede. So i`m asking, what kind of guideline can I use to solve this?

I believe that there's usually some degree of penalty for failures. And/or some sort of time-dependent random encounter table.

Also, if you spend enough time wandering around in an area, you will eventually find whatever it was you were looking for in it. The only question is, how long did it take to find and what happened to you while you were wandering around lost while looking. And, of course, with enough bad rolling, the players/characters may decide to pull back for now and make another attempt later, perhaps with more preparation.

I just have them make some rolls and create some time element for failures, and when I say time element it may very well mean they don't find "X marks the spot" until the 4th time they visit this system. This is especially the case when dealing with derelict space stations in a large system.