Hi Granny H. I've always replaced my board when I can no longer see the lines. It's usual that over time you literally cut the lines off. BUT, don't discard your board. You can use the back side for cutting too, especially the type of cutting that doesn't need lines. In fact, I often cut my batting on the underside of my board, so's not to ruin the lined top. No matter how hard you try to avoid it, you end up embedding batting into your board, when cutting with a rotary. So use the back side of the board. Will save you heaps!!
Cheerio for now