It depends what you mean by your line. If you just mean rats you’ve bred then sure, the first time you mate a buck and a doe together and they successfully produce kittens, you have a line.
To me, someone has a line when they’re producing litters of rats that strongly resemble each other, that have a recognisable look and consistency to them. To do this involves a few generations of careful breeding – maybe you start with rats from one breeders line, maybe you are mixing rats from different backgrounds. But you’re selecting hard for something in particular, and you are producing results – then you have a line, before that you’re breeding rats that are from someone else’s lines.
Some people can breed for two or three generations and have a real line of rats. Some people can breed for many years and just have a hodge podge of rubbish that I wouldn’t consider a line.