A word or short phrase instead of "the time taken between xxx and xx is..."

In looking for a word or short phrase to replace "the time taken in between milestone A and milestone B".

For example a project plan has 4 milestones:

Milestone 1: project initiative
Milestone 2: submit proposal
Milestone 3: deliver a working prototype
Milestone 4: submit a business plan

Different projects take different time in these milestones. Project A may take {12,10,5,8} days. Project B may take {20,6,15,4} days.

The days taken might be affected by human (e.g. forget, too busy etc), I am looking for a phrase to embody this.

So when describing the time (days) taken in between the milestones, what is the good and simple way to say, instead of "the days required from project initiative to submit proposal is 12".

Thank you.