Hi,
I checked few things a t my end and you can do it using workflow but it will be little complicated.
First create one custom field in opportunity entity called "counter", it should be integer type field.
Then create a workflow to update this counter every 24 hours. For this you can add a wait condition and a update step in the workflow.
Then the 2nd workflow, it will trigger on modify date field change and update the counter to 0.
Now you create a third workflow which will monitor the counter field value and if it is equal to or greater than 180 then you can send email and if the value is below 180 or above 365 no email should go out.
This is some ad hoc process but you can try this and verify.
Thanks,
Soumen
