Agree with everyone else that one long period is much less of a concern than lots of odd days for different reasons.
If I were recruiting and saw an application with that amount of sickness absence I would query it, I would ask the candidate what it was and what the current situation is. If I liked you and you were able to reassure me that it is dealt with, you are committed and enthusiastic etc then it wouldn't be a problem. You would probably have a medical questionnaire before you started work anyway.
An estimate would be fine, they will check anyway with the reference, but I would check anyway if you think there is any chance it may have been recorded incorrectly - you don't want a reference to go back to prospective employers with inaccurate absence information on it.