Usually I know how long I am used to riding and I look to push that distance until a 40 mile ride is "normal" meaning it won't kill me. From there that will be my minimum long weekend ride and I will push for longer distances.
So... my longest distance is 27.5 miles pulling my camping gear and it took about 2.5 miles.
On Sunday I am thinking of going for 3 hours regardless of how many miles I go. The other thought is to guess my average mph and go three hours worth.
No matter which I do I will get in some good riding and move towards my immediate goal of doing 100k distance.