I have steel rounds on a J-Bass (5-string) which were acceptable for 12 months moderate use, noticeably bad by 15 and replaced at 18 when the money finally came in to pay for a new set. I'm always slightly wary of changing strings too soon ("what if I spend £50 on a set I don't need yet?") but your gut is the thing to follow - it will tell when strings are dead - you just have to trust it.
I agree with the comments about SS being scooped - I use that bass for playing slap and it's set up for that exact set of strings, will always wear that exact set of strings whilst I have it and will always make a decent slap tone with a nice low action because of it. But even after 18 months wear and a noticeable dullness in the tone, all it takes is an EQ pedal (or in my case, an EQ block on a Helix) and it sounds like it has new strings again. You can do the same to a set of nickel strings or whatever (harder with flats I admit) and get what you want. The main thing is it has any strings at all - after that, most of what you want can be done with judicious EQ.
All this stuff about fret wear and killing fingerboards though... Look - one set of steel strings for 12 months isn't going to wear down your frets or your fingerboard. By the time you start to notice any consequences of using steel strings, you'll be 5, 10, 15, 20 sets in and unless you're gigging 5 times a week, many years into playing that particular instrument. After that amount of time, a fret job or an upgrade to something nicer probably something you're thinking about anyway. Other stuff about your bass will probably wear out first and provoke you into replacing / upgrading it before steel strings kill nickel fretwork - if it is nickel fretwork in the first place.
