You should have a measure field on Text. All you need to do is click on Text, the ellipsis next to the popup (...) and then type whatever you like in the box. You likely have SUM([SomeMeasureHere) or something similar - just type "days" after that.
I might recommend approaching this as a string.
Create a new calculated field - let's call it "Measure with Days"
STR(SUM([Days])) + ' Days'
Where SUM() is your aggregation, and [Days] is the field returning the number of days.
Give this a shot, put it on your viz, and you should be set!