I work on a team of three that has a project slightly larger than you 100k-110K LOC (and growing by about 5k LOC a week), we've managed to keep test coverage at about 95%, and have found it's worth the investment upfront, as it makes refactoring so much easier.
Looking back, I don't think even for a personal project, I would ever do something that wasn't a one-off without good test coverage. It's essentially taking on technical debt, as it makes you much more afraid of fixing anything.
Looking back, I don't think even for a personal project, I would ever do something that wasn't a one-off without good test coverage. It's essentially taking on technical debt, as it makes you much more afraid of fixing anything.