This is very basic stuff and you haven't given much detail so I am assuming you mean an Australian PPL(A).
The average course will depend on the minima set by CASA but you should always budget for another 10, maybe 20 hours more depending on your ability, instructor, weather, aircraft type, frequency of lessons etc. In other words, the minimum is exactly that, a minimum and there's no such thing as "average" in this context.
Yes, there will be theory and yes, it is part of a PPL. How much and which subjects depends on your national aviation authority; your instructor should be able to answer those questions for you.
Navigation is done as part of a PPL course and there will be an element of solo cross country flying. The skill test will also possibly include a navigation section as well as emergencies and handling.
If you've already started flying and/or have enrolled on a PPL course, I am disappointed that your school has not gone through all of this with you; especially the theory and exam requirements. Your school and your instructor should always be approachable; there is no reason why you shouldn't be able to ring them up and ask these things - they are, after all, the ones best placed to answer your specific questions.
Cheers
Whirls