Route plotting can be up to 1,000LY. Longer routes than this must be done in stages - plot your first route and fly it, plot a second when you get to the end of the first, etc.
Don't forget that you can plot either a route that uses minimal fuel, or one that takes the straightest line, both depend on your fuel capacity, jump range, and ship mass. If you plot a route and then take on cargo, you change your ship mass and invalidate the route. You'll have to re-plot to find out if you can still do it - or set the slider to the far right so it plots a route with your ship's maximum mass [all current equipment, ful fuel tanks and cargo racks]
As long as you can get to stations regularly on your route, or preferably have a decent fuel scoop in your equipment and a reasonable fuel tank, you will be fine.