If you're planning to maintain and develop your code base it'll be worth it no matter how long it takes.

You'll eventually find yourself modifying your application so it's easier to write tests for.. this is a good thing. It'll probably mean you're making your code more modular which will in turn make it easier to develop.

oh, and you'll know it was all worth it when you make a quick and simple change which "won't affect anything else" and your test suite finds and identifies a bug in 10 seconds..

I've spent a lotta time writing tests and improving my app to make it testable over the last 6 months, but I still don't think I've come close to the amount of time I've saved compared to manual testing and searching for bugs...