pinging for 15minutes seems like not a good idea, could have some side-effect which i am not sure of. besides ping delay is not really accurate. that's why in the first place i suggest using vbs sleep function instead of ping.
if you just need to delay for 4-5 seconds once in a while, i would have suggest you using ping instead of vbs sleep.
the code:
>$sleep.vbs echo wsh.sleep 900000
cscript//nologo $sleep.vbs
900000 is in miliseconds, or 900seconds=15minutes.