forms – Best user experience when asked to enter the frequency of the periodic event

So I'm working on a form and one of the entries has proved problematic in user tests:

We want to save the frequency of a periodic event, the event is the frequency with which a report should be sent, in our database we will keep a numerical value that describes the hours per year.

It can be more or less once a year, so in the case of every four years, the value we want to save is "0.25" and for four times a year it is "4".

In a previous form, we ask the user to calculate the number by themselves; In use, this has proven to be difficult for users.

We have tested a solution with two input fields, one for the frequency and another for the year: X times per year Y (where X and Y are the input fields). In a user test with 4 users, we only had one solve it relatively without problems and 2 did not solve it at all when they were asked to enter "once every two years".

Samle entry form when the entry is once every two years

Any suggestions in other ways for the user to enter the requested data?

Thank you,
Roy