The trouble with using a 555 for a long timer is that large RC values must be used. There is an upper limit of a few dozen megs on th R because of PC board surface leakages which means the C has to be large.bogdanfirst said:well, it is a timing problem if the surrounding temp variates much.
but also if the IC or whatever you have is heating. but i don't think that your 555 will heat too much, since you will probably connect the load trough a transistor and/or relay so you will not draw much current from the 555.
if you use a 555 as an astable mode, the HIGH time is t=1.1R1C, where R1 is from Vcc to pin 7.
so if you try to time something like 2 hours=7200 seconds, you will probably need something like this : C=4700uF, R1=1.39Mohm, or.....
you realize that the values are quite big.
think that the value of the resistor is so big that if you were to accidentaly touch it, the timing will drop quite a lot.