Learn one module deep (call it core competency module) and branch off to other modules interfaces. Then take up new project module implementation on which interface is learned with the core competency module. Also understand the source code with a little ABAP training esp. writing user exits/CMOD, debugging etc. Keep your interpersonal skills cordial amound the team and be honest with the project time billed. Do not bill the project for the extra time spent to learn some thing new as that is not a deliverable to the project. This will keep you in the market for long time and hardly any worry on being waiting on bench.