you will tire easily if you fret down excessively (regardless of sitting/ standing). the common perception here is to press down hard for better effect but the key to playing well & helping you focus is to fret down as hard as it takes, nothing more.
this brings me to the picking hand- if you pick hard, chances are, you fret down equally hard. beginners tend to do this excessively which isn't necessary, pick & fret sufficiently; as long as there's sound coming from your guitar (manifested by the amp).
how long does it take before one discovers this 'perfection'? Answ: as long as it takes. every time you play, make a conscious effort to ask yourself- am i picking/ fretting excessively?
remember: excessive force = lethargy = mistakes = motivation killer