Kenneth Fisher recommends cycling the SQL Server error log regularly:
You’ll note I have it set to keep 32 files. This way I have at least a months worth of information. It’s probably more than I need, but for my personal box, that’s what I chose. For your company, it’s going to depend on your needs. You can keep up to 99 files but that seems a bit excessive to me. Of course, these files aren’t all that large (usually) so it’s up to you.
I’d go with a bit more than 32—if you have three service restarts in March, you won’t have a full month’s worth of information. Regardless of the exact number you choose, it’s a smart idea to cycle this regularly.