Trimpot potentiometer rotational life Vs load life
The two main failure modes of a trimpot are ...

a) the track wearing away, from excess mechanical use, leading to a high resistance and noisy connection track to wiper

b) contaminants interrupting the contact between track and wiper, leading to a high resistance and noisy connection track to wiper

Most use, yours included, can probably ignore the first problem. The second problem has two main components, Large particles, like dust, and reactive contaminant gases, like sulphur and nitrogen compounds. These will generally react faster when hot. As you are going to be running your trimpot cool, you might like to hope that you would get more than 1000 hours out of it.

The connection you've shown is particularly nasty in adjustable voltage regulators, as the loss of track contact gives high resistance and therefore high output voltage (the nastier type of error). You can reduce the amount of damage such a fault can do by connecting the wiper to the unconnected top of the track. That way, if the wiper loses contact, the resistance only goes to 1.5M, not to infinity. 

A capacitor ADJ-> GND (up to 1uF permitted by the regulator data sheet) will reduce the size of the positive excursion that any momentary losses of wiper to track contact cause as you adjust the setting.

It's worth bearing in mind that a trimpot is going to be orders of magnitude less reliable than a soldered connection to a resistor, and possibly an order of magnitude less reliable than a large potentiometer, so try to avoid them if you can.

Ideally, in this sort of application, you would not use a trimpot. If you want to set the voltage carefully to a setting and leave it there, then it's worth soldering in a resistor, measuring the voltage, calculating a new value, rinse and repeat. If you want to adjust the value often, then get a 'real' 1/4" shaft potentiometer, or an IC 'digipot', that's programmable from a microcontroller.