Unless you have an extremely powerful processor this takes a lot of time.
Not to mention, if you don't even know how many characters are in the password, you'll have to try every possible combination of different character lengths too. This can take (literally) years on a single processor. The most efficient way would be to use a server cluster (hundreds or thousands of systems) all trying different different passwords within a set range simultaneously. Depending on the amount of processors trying different passwords this could be reduced to minutes to even seconds.
If you have a quad core processor, and know what you're doing, you can write a bash script to utilize the cores you have to continue trying different passwords on each core. This can further reduce the amount of time it would take.
Though, personally, whatever is in that rar isn't worth the effort.