A lot of times you want to describe the purpose of the 15 minutes in some way. When the purpose is described by a noun, the adjective **15-minute** can be used. For example:
> a 15-minute break

> I'm taking the dog for a 15-minute walk.

In Dutch you might multiply the _kwartieren_, for example to denote 45-minute or 75-minute periods (30- and 60-minute intervals are more commonly referred to as half-an-hour or hour, respectively). In the 45-minute case, the adjective **45-minute** seems pretty common, for example:
>a 45-minute class

>a 45-minute lunch break