This question can be answered two ways. The acidity of lemons is due to citric acid. The amount of citric acid in a lemon is about 5-6%. If you are asking how strong is the acid in a lemon, then you are referring to the pH scale. The pH scale is a measure of the hydrogen ion content in a solution. The scale ranges from 0 (very strong acid) to 14 (very strong base) with pure water at a pH of 7.0. Every change in step represents a 10 fold change in the strength of the solution. Thus, a solution with a pH of 2.0 is ten times stronger than a solution with a pH of 3.0. For a lemon the pH ranges from about 2.3 - 4.0.
It's best to check the acidity of lemon juice using a pH indicator, preferably Litmus Paper. You can ask for some from your science teacher, but I'm pretty sure lemon juice has a pH of 2 or 3.
Hope that helps! :)
** by the way, pH measures whether a substance is acidic or not. (a non-acidic substance is called a base)