It depends a lot on the ambient temperature. If it is above 30ºC, consider it to be unnatural, but always check the exhaust air condition and inject cold air.

Remembering that the most efficient system would be that they expel the air in the open, however as we work with electronic components this becomes impossible because of the rains, but a draft is always welcome.

The good thing would be to put in an environment like I said earlier with constant or forced air exchange.

Spacing between GPUs is also important.

Standard temperatures for

R9 280x is 68 ° C ~ 75 ° C
Rx 470 4gb with Oc, 64 ° C ~ 70 ° C
In Rig, Space 8 cm