How often does the weather condition have to happen over the time frame for my ads to trigger?

When you select a weather condition as part of your weather targeting rule, the default 'frequency' setting for the condition is 'at least once'.

This means that it if that weather occurs at least once, any period within that time frame, your action will be triggered.

So for example, if you trigger was 'any rain', and your time frame was 'in the last 3 days' [-3 days to now], any rain in the last 3 days will trigger your associated action.

However, the frequency setting can be changed in the 'advanced options'.

Read: What are the Advanced Weather Targeting Options and When Should I Use Them

For example, you can choose to trigger ads if it has rained at least once on each of those days, or continuously over that whole time period.

This means you have ultimate flexibility in setting up extremely nuanced targeting rules that best fit your campaign objectives.

Further reading: How time frames work for weather targeting ads