If you are using TNG 2.x then you need to alter this file on your DSM server: tng\services\config\aws_nsm\dm\ping_22.dat
Look for this section and change the poll interval to whatever you want:
#moclprop
moclprop_class      :Agent:Ping
moclprop_index      :NextIndex
moclprop_prop       :pollInterval
moclprop_value      :300

If you are using TND 3.x / NSM then look for these files:
tnd\services\config\aws_nsm\dm\ip.atp and pingip.atp
(IP.atp is for hosts / workstations. pingip.atp is for unclassified objects and printers etc)

Look for this line and change it:
int     pollInterval            = 900;

After - you will have to run a resetnsm or resetdsm

Sorry, I should have mentioned. If you change the polling via this method it will change it for all objects - rather than just for the unclassified class.
If you want to change it just for the unclassified class then you will have to do it through pollsets.
In pollset browser - give it a name in the "name" field.
From the Agent drop down choose "Ping".
From the Host dorop down choose "unclassified_Class" - "Unclassified_TCP"
Click the "+" button.