I'm not sure there is a recommended length as it depends on where you tie up and what to and to a certain extent what attachments you have on your boat - some have remarkably few!
On my 5.8m I made up my own lines out of 10mm white nylon rope, bought in bulk and cut to length and spliced myself, 3 strand rope is easy to splice yourself and a skill worth learning. This has a breaking strain of 2000kg which is plenty and gives a bit of stretch if you find yourself bouncing around a bit.
I have seven main attachment points on the boat;
- Bow eye
- Two cleats on the tubes forward of the console
- Two cleats at the rear of the tubes
- Two cleats on the A frame
As I run around in mine with lines attached all the time, I have sized the painter and the two forward lines to be all just too short to reach and tangle in the prop if the loose end becomes detached and falls in the water when underway - I made them about each a foot too short so the painter on the bow eye is about 5 metres and the other two are about 4.5 I think.
They are secured when underway but this guards against the obvious possibility of one coming adrift and getting your prop tied to the front of the boat while underway and being unable to tilt the engine up to do anything about it!
Obviously you can't do this with the stern lines or they would be useless but being attached at the stern they are less likely to get sucked under the boat so those are about 5 metres on mine.
You may also wish to have a "tow line" in case of breakdown, in my case I would intend to use the anchor line which is 50m of the same rope, but I also carry about 20m of rope in the console just as an extra.
That's just what I've done, as I say I don't think there is any right or wrong answer but the bigger variety of ropes you carry the more likely you are to have the right one for the circumstances