i find that most every spell is different, each will have a different outcome, different length, different power, different starting times, etc. it really depends on the spell, the belief in the spell and the power put into the spell. sometimes the effects may wear off quicker than others, sometimes because of a low level of energy or will in the spell.
i usually take it by feeling, if i feel that the spell's strength isn't strong enough anymore i might recast it. certain kinds of spells -like banishing negativity, cleansing and protection spells - i try to do at least once every two weeks to be sure.
since you said you were new, take sometime studying and meditating, learning how to properly cast spells and build energy then you'll gain a sort of "sense" to when you need to do a spell again.