Does the motion sensor has a selectable timer (e.g. 15-20 minutes)?

No. All our motion sensors do not have timer.

In the SK698 and SK602, once the motion sensor is triggered, the receiver will sound the alarm for 10 seconds (except Ding-Dong sound). 

In the SKQH652, the alarm will sound for 25 seconds. The motion sensor detection interval is 5 seconds.

Please take a look at the SK633SK634, and SK618 if you need more advanced features.