It’s a little complicated.
In general, automations trigger the moment the condition becomes true. For example, if the trigger is if a field has a value, the automation triggers on the first character entered, even if the user presses more keys right away.
In general, formula fields recalculate with each key press. However, some very complicated formulas can take a while to recalculate.
The notable exceptions are when you have formula fields that include
TODAY(). Those do not recalculate continuously. How often
NOW() recalculates depends on whether or not someone is viewing the record. It can recalculate as soon as every few minutes or there could be an interval of an hour or more.
Thus, if your automation is based on a formula that includes
TODAY(), timing of the execution could vary from immediately to an hour or more later.