There are very clear Rrotobrush tutorials on the web. Go through them. Unfortunately I'm not in a position this AM to do a search and give you links. Rotobrush is not intuitive. You have to go through the tutorials.
If you go back you should not loose work up to the point where you make changes but it makes complete sense to have to redo the work that follows. Make sure you are carefully watching the progress and stop often to add new strokes. Don't make too many. The fewer the better. Basically you should extend the span to make it longer. It's the bar below the footage timeline with the > > > marks. Make your first selection, make any corrections to the first selection, then press the space bar to propagate the roto. As soon as you see the mask start to drift press the space bar again, back up to a frame where the selection is still accurate and add or remove area from your selection. Press the space bar again to move forward.
If you are at 2:20 in your 3:00 clip and you see something that you missed at 0:10 then going back to fix it will mean you'll have to propagate all the way back to 2:20 to get back to where you were. That's just the way it works.
Good luck. A 3:00 roto job with any software is a daunting task. I'm not trying to be critical but I don't recall ever needing a 3:00 shot to tell a story. Getting yourself into this kind of a post production problem means that there was something lacking in the production of the shot... Planning. Fix it is post is an expensive and time consuming proposition. I wish you well. Maybe there's a better solution.
Thank you , I have notice there a light green line (not the smaller green line showing the progress ) just above the timeline with the >>> , more you progress in Roto brush more this light green line decrease , if this line dissapear you lose all your roto brush ,so it´s telling you aproximately how much you can roto. I am using CS5.5. Basically I have done it in 4 parts , aproximately 40 seconds each part of roto , then I regroup all parts with premiere.