I'm a solo developer with a pretty time-constrained work environment where development time ranges usually from 1-4 weeks per project, depending on either requirements, urgency, or both. At any given time I handle around 3-4 projects, some having timelines that overlap with each other.

Expectedly, code quality suffers. I also do not have formal testing; it usually goes down to walking through the system until it somewhat breaks. As a result, a considerable amount of bugs escape to production, which I have to fix and in turn sets back my other projects.

This is where unit testing comes in. When done right, it should keep bugs, let alone those that escape to production, to a minimum. On the other hand, writing tests can take a considerable amount of time, which doesn't sound good with time-constrained projects such as mine.

Question is, how much of a time difference would writing unit-tested code over untested code, and how does that time difference scale as project scope widens?