Anyone???

I use this to change an image on the 1/4 hour

Dim curTime As Date

curTime = Format(Now, "hh:mm:ss AMPM")


'change picture every 1/4 hour

If curTime = "08:00:00 AM" Then Call cmdRandom_Click
If curTime = "08:15:00 AM" Then Call cmdRandom_Click
If curTime = "08:30:00 AM" Then Call cmdRandom_Click
If curTime = "08:45:00 AM" Then Call cmdRandom_Click

I would like the user to be able to choose between
10 seconds, 20seconds, 40seconds, 10 min, 20, min, 40 min

one problem....
I haven't a clue how to go about this or even if it can be done.