I have been in similar situations when I was going on a trip on a tight budget and was having trouble putting together a budget. My solution was to wing it to a degree. Yes, you still need to do everything you can to keep your cost down such as camping, grocery food, etc, but don't set a definite end date. Instead, set a specific dollar amount that ends the trip. For example, you may set $1000 as your end point. Once you only have $1000 left, its time to start heading home. $1000 is just an example, but you can figure what it would cost to get home from the farthest away potential area (add some cushion!) Hope this helps.
EDIT: sorry, just realized this is a sticky and not a recent thread