Thermostat

If the thermostat housing is leaking then replace the gasket. A good brand thermostat can last years. You can test by placing in water heated up on the stove to see it open up or not if bad.

Isn't it supposed to have a seal on it that you have to put that sealant

It has a rubber molded gasket. I do not think it has to have any sealant. But if you wanted to just on the flat surface (not the recessed area) you could use some old style copper gasket maker (not silicone) let it get tacky before assembling. Should not get any one thermostat and put very thin solid layer to both surfaces (housing & intake flat surface only whip off excess). Let it air dry just until you can touch it and it does not stick to you. Ask the store clerk for the old style (kind we used to make our own gaskets with back on the 1980's and older cars, they still sell it. Its copper/brown color. But like I said you should not need it.

I looked gasket up and its shown it can be purchased without the thermostat if yours is good.

How often are the thermostats supposed to be replaced

Sorry, thermostats do not have do not use after this date shelf life. What I would do is change it when you change ANY radiator or heater hose. Unless it gives out sooner. If you keep your cooling system flushed and maintained, the thermostat should last a very long time. Example I have a 98 truck I just replaced mine a few months back (water pump too), I keep my coolant fresh by changing it every year. My 19 year old OEM original thermostat is still good (tested). I put it in the Oem replacements box (as I buy Genuine Factory parts when those parts do not have build issues) to save in-case my new one ever gives out I have it to use in a emergency. If your coolant is dirty and you flush it and it returns back dirty in short order then its time to replace it or if you change your water pump its time to replace it. If your concerned about it replace it But I advise as important as a thermostat is I advise the purchase of OEM parts as much as possible (they last longer), if yours has a 2 buck thermostat, Replace it with stock oem parts. If you do not know which you have, have a look and replace it while at it. It is a part that is replaced when coolant systems are serviced with other new parts, it just the way it is. Your cooling system needs flushed and changed just like oil & oil filter if it has not been the life of the thermostat is greatly reduced.