I have played around with this quite a bit as we have use of an idoor straight and havee had to split longer runs down to fit in there at times.
I have found that with anything up to 250m (total distance) then 60sec and a 15m roll in for second run and any subsequent runs to equate nicely in terms of total time.
For 300m + then 30sec with a 15m roll in for second (and third) reps fits well to get equivalent times if you were to run the rep as a single run.
Obviously if you want faster than the single run then open up the between runs recovery a bit.