It's impossible to predict the unknown. The best you can do is limit the number of bugs that get through, by rigorous testing at each phase of development. Every function is considered a potential bug until tested for all possible inputs (min, max, a random middle case if a range of inputs, all inputs if a finite non-sequential set). Every routine using an untested function is considered a potential bug until all functions it uses are tested and the routine itself is tested. Every section of the program using an untested routine is considered a potential bug, etc. etc. Testing takes time, but in a large project that's more than made up for by time saved not having to fix bugs that could be anywhere inside hundreds of thousands of lines of code.

Basically, your best bet is to read up on how to properly test your program and prevent bugs. You limit disease through proper hygiene, not by worrying about how you're going to stock a cure for everything on the face of the planet. If a bug DOES make it through your testing procedures intact, estimate time based on the programming phase you're currently in (a bug in a routine is far easier to locate and fix than a bug somewhere in a "finished" program) and don't sweat specifics. Eventually you'll have enough bugs from each phase so you can fairly well average based on past performance how long it's going to take.