Rapid rise in temperature in infants, toddlers, and young children can cause febrile seizure. Given the trade-offs, we generally choose to medicate our child when his temperature passes ~102 and it's on a rapid uptrend.
As often as you need when they have a fever. Modern thermometers can give you the temperature in a minute or so.
There should be no major variation then. On the other hand, if the child reaches 38+ or so, you need to be on the lookout...