Average months between missing and hitting SLA

Hi, 

I'm trying to work out the average periods (months) between missing SLA and hitting SLA. My data is refreshed monthly. For example; if a campaign misses in January 2019 and hits in February 2019 then the average period it takes is 1 month. 

I need to take into effect that a campaign might miss their SLA in January, hit it in February - which would make it 1 month. The same campaign might miss again in May and June and hit again in July - which would make it 2 months. The average then for this campaign is 1.5 months. 

I also need to take into account that if a campaign misses in June and hasn't hit SLA again since then the average of that period would be a datediff from June to now. 

The fields in my database are;

- Date (date)

- Client (nvarchar)

- Actual (float)

- Target (float)

- FTE (int)

Many Thanks!