This is a difficult question to answer as there is no one right answer. However, here are some basic ways to begin narrowing down an affordable place (note: affordable differs from person to person and we are only giving the generalizations here).
First, it obviously depends on where you travel, some regions of the world are more expensive than others (think Scandanavia vs Southeast Asia). As a result, your home currency will last longer in the less expensive regions.
Second, the type of traveler you are makes a huge difference. Luxury travelers will have a completely different budget compared to backpackers. Then there are those of us who fall into neither extremes.
Third, when you travel also makes a huge impact on cost. Summer vacations (when students are generally off from school) as well as holidays tend to be higher than off-peak seasons (this varies place to place).