How do I keep my bed heated for 3+ hours to dry filament?  

That's a safety measure ... and it wouldn't do a very good job of drying in any case.

Heating doesn't dry anything, it just makes the moisture more mobile and hot air can carry more water vapour - the relative humidity drops as the air warms so it has more 'drying power', the trick is to move the air away as soon as it has absorbed all the water it can carry and replace it with dryer air...  you need a closed space with a gentle warm-air fan.

At  pinch you can use some domestic fan ovens if their temperature scales go low enough but you have to leave the door open a crack for the moist air to escape.

Most of us use either dedicated filament dryers or, often better, food dehydrators (with the side benefits of being able to make tasty dried food snacks in season).

Your print bed does easily reach the required temperatures but it doesn't have enough power to continually heat fresh air for fan-blown heating.

Cheerio,

RE: How do I keep my bed heated for 3+ hours to dry filament?

Diem has explained why a proper device for drying filament is better.  However if you still want to heat your bed for 3 hours then you would have to run a gcode file to do so.  While its running the file its 'active' so the safety timer doesnt kick in.

Something simple like

M140 S40 ; set bed temp to 40 degrees
G4 S600 ; wait 600 seconds (10 minutes)
M140 S40
G4 S600

repeat those in a gcode file  as many times to get to the time you want to heat it for.  You have to do it in smaller time slices as otherwise the safety timer will kick in. 
Then add at the end M140 S0 ; turn off heatbed,  to stop it.

RE: How do I keep my bed heated for 3+ hours to dry filament?

Thanks!  I actually have a Polyphemus on order, but I had some important PETG parts to make for my friends immediately.