Minimum 35 minutes for the densest routes in the game, 60 minutes being the rule of thumb. The penalty is your flights are viewed as one flight versus two, eliminating the frequency bonus.

Another related question:

Is the allocation algorithm a "sliding" one or a "binary" one.

For example, say the route is not so dense, thus 60 minutes is the optimal separation. Do all the separations in five minute increments from 05 minutes to 55 minutes result in the same allocation?

Or, in this example, is the allocation on some scale that recognizes the degree of separation such that 05 minutes is the least and 55 minutes is the most, while 30 minutes is the mid point? If so, might be much to ask, but is it linear or curved (flat, very little change close, with most of the change at the end of more separation)?

I ask because if it is binary at 55 minutes, then I might forgo that time slot for another combination. If it is sliding, it might be a better fit all else considered.