Truthfully, the infinite incubator is and always will be hatching an egg. Every time something is incubated with a usable incubator, you as a player, always save yourself the great distance of walking. It is better to think about the incubator as a kilometer saver.
For the incubator to be most effective, you can use it on three 10k eggs. This means that you will save 30km of walking. Thus, incubators can stay in the inventory for as long as you like. It is most resource-efficient to incubate 2k and also 5k with the infinite incubator.
This is quite important now, all because the super incubators became available. The super incubator has almost the same function as the ordinary incubator. If you use the super incubator on three 10k eggs will take 20km walking to hatch, but nonetheless both of the incubators save 30km of walking. This means that the super incubators in the special boxes are exactly the same as the regular ones.
Let’s say that, in the future Niantic was to change the system to include some incubators which are based on distance or time, the most effective way to hatch eggs will still be the habitual incubators.