How to calculate the Temperature rise of PCB and its surrounding air from the power PCB consumes?
The thermal time constant of 1 square of PCB GROUND plane, of size 10cm (4"), is 96 second. The time constant of a square meter of foil is 9,600 seconds; of 1cm is 0.96 seconds; of 1mm square is 0.0096 seconds (9.6 milliSeconds).

If we model the heat flow as from one edge of the foil, thru the 4" of foil, and exiting the opposite edge, then that square of foil has a Thermal Resistance of 70 degree Centigrade per watt.

Thus worst case temperature rise, with 1 watt of heat generation spread along one edge of the PCB, and the heat flowing only through the foil (no heat exiting into the air), to exit into the air (or thru metal mounting posts into the case) at the opposite edge, will be 70 degree C /Watt * 1 watt = 70 Deg C.

However, if the heat has to flow through long narrow traces to reach the Ground (or power) plane so the heat can spread out, then you can have much hotter local regions, and the slowly moving air flow will easily cool regions that are 100C or 150C hotter.

SUMMARY: 

reach thermal equilibrium? yes, with one TAU being 96 seconds.

assume uniform heating on the PCB? at 70 degree C per watt of heat flow? not likely. You need at least ONE plane, to really spread out the heat, or a bunch of WIDE traces exiting your hot-spots.

That 70 degree C per watt is PER SQUARE. A trace 2cm long and 2mm wide has 10 squares, thus is 700 degree C per watt. Use a plane, or wide-and-short traces. And you need to dump heat THRU the FR-4 epoxy fiberglass substrate, to move heat into the plane.

Draw some sketches of the PCB heat flow, using a bunch of resistors to constrain the heat movement.