If you’re not looking at any particular time target I would say something like 8 weeks. Essentially 6 weeks of increasing long runs, adding 1-2 miles each time to a weekly long run would comfortably get you to 20m or so which is enough, then a couple of weeks of tapering down.
A lot of the generic marathon programmes are set at 16 weeks or thereabouts (New Year to April to help the VLMers for example) with a half marathon distance approx half way through so you’re already comfortably at that point. Your aerobic endurance will already be really good from running long distances as much as you are so the step up to a full marathon won’t be too difficult.
You could do it in less with your existing base frankly but 8 weeks would be sensible and less risky from an injury perspective. Remember to run a bit slower than you would for your current distances if going longer, and also think about fuelling - it’s easy to run for a couple of hours with no calorie intake, but anything longer you will really need to take something on (whether gels or ‘real food’ is up to you, experiment on your training runs to see what your stomach can cope with and what works best).
