Some people build up a tolerance to the drug, some do not- it's different for every person. Body weight is not taken into account because everyone reacts differently to the medications, for example a 90 pound woman could need 60mg to see effects but a 200 woman could respond to 25 mg. It has more to do with the brain than the body size.
My personal experience has been that I start to develop a tolerance after 3-4 weeks of taking it daily. I decided that in order to avoid taking a higher dose (because I didn't want to deal with side effects- I have none on the dose I take) to only take the adderall "as needed", on days I have to get a lot done or need to concentrate more.
Just keep in mind that adderall is not the only med. available- there are many to try if it's not working well. Also, stimulant medication are not addictive at the levels prescibe by a doctor to a person with adhd. They can be abused by using them improperly (that's why they're controlled), but do not confuse them with the street drug "meth" which is a mixture of highly toxic ingredients, only one of which is shared (and not even in the same form) with the legal medications.