I heard that Arjuna was not together with his brothers for 12 years when he was exiled and later he joined them in the 13th year of incognito exile.
So where he was for 12 years?
Actually Arjuna underwent two separate exiles. One was without his brothers for twelve years and the other was with his brothers for thirteen years. Some people may have intermixed the two.
The first twelve years of exile he faced was because he had to break a mutual agreement that the Pandava brothers had among them and the second thirteen years exile was due to the defeat in the dice game.
Reason for Arjuna entering the twelve years exile:
After the five brothers married Draupadi, Narada visited them and said they should have a mutual agreement of how they will spend time with her. Narada told a story how two invincible demon brothers lost their life fighting over the possession of a apsara. So the five brothers made a rule that they would spend time with Draupadi in turn and no one would interfere while any brother was with her. If anyone did so, then he would have to enter into exile for twelve years as a celibate.
But one day to help a Brahmin save his cow Arjuna needed his Gandiv dhanu for which he had to enter into the room where Yudhisthira and Draupadi were together. After returning Arjuna asked permission from Yudhisthira to go into exile. But Yudhisthira said it is not necessary because his intention was good. However, Arjuna to keep the promise and respecting truth entered into a 12 year exile.
During this 12 year period he wandered many places, took pilgrimage, spend time with sages, married some damsels, spend time with Krishna, abducted and married Subhadra and then returned to Indraprastha. So some of the events are as below:
Thus Arjuna spend his first twelve years in exile.