Well... there is a lot of things involved in the high.
First, there is the amount and concentration of the active substances (usually THC/CBD) in the weed. As larger it it, larger the duration of the high.
The intensity of the high (level of highness) also increases with the amount of THC/CBD, but only until a certain level. Beyond this level, an increase in the amount of THC/CBD wont make you any higher, but will only make the high last longer.
Also, there is the interaction between the cannabinoids in the weed. Some cannabinoids (usually CBN/CBD) can make the THCs effects last longer, so strains rich in CBN (and sometimes CBD) will take longer to kick in ("creeper" weed) but the high will last longer.