I want to turn my Heaters on manually but then have them turn off after a certain amount of time (eg 1 hour) automatically. Any idea how I can achieve this?
It looks like you need to dive into webCoRE 
Once you have it installed you can create an IF/THEN piston to get this done.
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Do you use webCoRE? It’s fairly simple piston , if heat turns on, then wait x number of minutes, then turn off
Beat me to it, couldn’t edit my post fast enough
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You can certainly do this with webcore, but if you prefer, you can also do it with the official smart lighting feature with the option called “power allowance.” 
And if you would like to sometimes be able to turn it on and have it stay on longer than an hour. And other times have it turn off after an hour, you can use a virtual timer.
There’s a how to article in the community – created wiki on those. Basically you still use the power allowance feature, but you apply it to a virtual switch instead of the actual switch. (although these articles talk about lights, they can apply it to any switch that is controlled by SmartThings.)
So lots of choices. In the long run, webcore will let you do the most, but it has much more complex set up. So that might be overkill for something this simple.
http://thingsthataresmart.wiki/index.php?title=How_to_create_a_virtual_timer_for_a_light
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