How long is "quite long"? 200 feet? 2,000 feet?
I think a well managed tow will be somewhat dynamic in nature, not a set it and forget it type of thing. Assuming a long tow for covering ground on open water; factors that may need consideration include weight and drag of the towed vessel as well as wind and sea state, as well as weight and construction of the tow line.
I think the goal is to have sufficient catenary. If there is enough catenary in the tow line, the weight of the tow line should work as a shock absorber, So however much line that requires in the current conditions. Plus, as mentioned by RichH, I think it would be a good idea if you were to sync with the length of the tow line with the wave period to reduce shock loads, and to not pull your buddy through a wave (which he may not appreciate).
The other consideration might be to bury the catenary below the surface of the water to reduce the impact of recoil if the line parts, the water might take some of the zing out of things.
A big consideration, especially if the tow is bigger than the vessel doing the towing which is often the case, is you want them far enough back that the towed vessel does not over take and GIRD the towing vessel. Girding is probably the biggest single risk in a towing operation. Basically, it happens when the tow overtakes, and side loads the tow line on the towing vessel (which is ideally attached forward of the rudder post, as Fast explained earlier with dinks). When the towed vessel over takes, it flips the towing vessel and possibly runs it over. Bad. Very bad. This is a big concern with following wind, sea or current, but should always be a consideration. You might want the towed vessel back far enough that you KNOW you can cut the line before the tow can over take you.
Go out and practice with your friends. It will be fun, just make sure you have enough fenders if you are doing a hip tow. Play with different tow lengths and observe. Look listen feel. I think a well set tow line will feel like a well set tow line.