The chunk itself is the easy part. You move $15,000 from the line of credit to the mortgage principal, the amortization schedule jumps forward, and it feels like something happened. It did. It also moved the debt, and now the clock is on the payback.
The window is set by one number: your real monthly surplus. Income minus everything that leaves. If that's $1,200 a month, a $15,000 chunk takes about thirteen months to clear, and that's your answer. Not a target you picked. An output of arithmetic you already did.
Longer isn't automatically wrong. The line of credit charges simple interest on the average daily balance, and the mortgage charges amortized interest on a much larger balance, so time spent carrying the chunk is usually still cheaper than the interest the chunk killed. The risk isn't cost, it's exposure. Every month the balance stays high is a month you have less room available if something breaks.
Shorter has its own failure mode. Squeeze the payback into six months by cutting everything, and month four brings a car repair, the surplus you promised isn't there, and now you're borrowing more on a line of credit you were trying to clear. Then the next chunk gets delayed, the one after that gets skipped, and eighteen months later the whole thing has stopped. Plan the payback with room in it.
A workable range for most households is nine to eighteen months per chunk. Under nine and the budget usually has no slack. Over eighteen and the balance sits high long enough that a rate increase or an income change catches you mid-cycle. That's a starting point rather than a rule, and your own surplus overrides it.
Size the next chunk off what the last one actually took. If the $15,000 took sixteen months instead of thirteen, your surplus is really $940, not $1,200, and the next chunk should be smaller. The line of credit statement tells you the truth about your budget faster than any spreadsheet does.
Don't wait for a zero balance to chunk again. Once the balance is low enough that a new chunk still leaves your safety room intact, go. Waiting for zero is tidy and it costs you months of progress on the mortgage side.
The one thing to hold constant is the payment itself. Whatever you were sending to the line of credit each month keeps going there through the whole cycle, including the months when the balance is small and it feels unnecessary.