I had originally created these respawning class targets when I created the map achievement-idle-spytech (I hate not being able to type underscores on this site).
If you've downloaded the map (it's available here on GameBanana, just check my profile), you'll notice that all 9 class targets are located in the "hidden admin room".
Extract the contents of this folder (minus the README, of course) into:
And you're done!
All 9 Targets have a respawn time of 3 seconds. To change the respawn time, insert the Target prefab into your map, right click the selected prefab and click "Ungroup".
Separate the 2 target props (as each prefab consists of 2 separate Target props and a point-template), then select both prop-dynamics (aka the target models) together, right click them both and click "Properties".
Navigate to the Output tab, and change the "Delay" value to whatever you want the respawn time to be.