I have a Burris FF3 which features an on/off switch and also software that turns the sight off after inactivity. Took it to Africa. You guessed it, lined up on a kudu only to find it switched off. Likewise found on several occasions that after being carried in a holster all day, it had switched itself off. The Burris now sits on a range gun where it doesn't matter.
That's one of the reasons I much prefer JPoint/Shield type sights, once exposed to light, they are constantly on and only switch off when in total darkness. I tested one by not changing the battery. It was a bit over 4 years before it stopped working. (only in use during weekends).
At the range it is common to see people have to stop and switch on sights.
Of course each sight is different. Work out what the battery life a handling features of your sight and be prepared. Establish a routine whereby you change the battery (and re-zero if needed) at regular intervals (for me, once a year).
When hunting you have enought to think about without worrying about whether the sight is on. Keep It Simple.