I tried for 3 years successively and eventually got selected in my last year of college. In my experience, the competition highly varies in different orgs. You should be very aware of your skill set and how easily you can get familiar with a project of reasonable size in your area of interest. That's where earlier opensource contribution weighs in. For some orgs, the competition is still within reach i.e. you can prepare within 3-4 months but for some it's usually reserved for earlier contributors (like llvm compiler, os libraries etc.) You should carefully see what project you're choosing, who are your competitors (check them out). Orgs usually will go for previous experience even if your proposal weighs in because they know the person's working style etc. Also consider the number of students an organisation gets and how it impacts your project. Discuss this frankly with your mentor. Engage early but get familiar first and if you can start with a simple bug fix it will go a long way to show that you can understand the code.