The length of your trip can be determined by many factors. If you are in school, you may be limited to a summer. If employed, you could be limited to a much shorter period dictated by the amount of vacation time you are allowed. If you are in your gap year between graduation and work, you can have a much longer period to travel, and maybe even go on a round the world trip. Although you may have the time, you can find your trip being shortened by the amount of money you have saved to allow you to go on such a backpacking excursion.
If you are traveling with a friend, you need to make sure you have spoken to all travel partners to make sure your travel time is in sync. If you find yourself with more time off than your travel companions, you can consider doing half of your trip with a friend and finishing your trip on your own. This way, you can have the best of both worlds, won’t miss out on all the places you wanted to go to, and will have a little experience under your belt so you won’t feel so clueless when you are finally solo.
If time is not an issue, then you can expand your trip by visiting cheaper regions. Southeast Asia is known to be one of the cheapest areas for backpacking. South America is also a good region if you are trying to save money, as well as Eastern Europe, although it is not as cheap as it used to be. I have listed other destinations, including those that are cheaper and those that are more expensive in my post about where to go.
Another way to extend your travel time is by splitting your destinations between big cities and smaller villages. The larger cities tend to be way more expensive than remote towns. Try to plan your trip by stopping in the smaller towns along the way instead of focusing just on city after city.