This will truncate the decimals:
toString(floor(hoursBetween(($currentObject/Expiration), [%CurrentDateTime%])))
or if you want to round to the closest integer, first add 0.5:
toString(floor(hoursBetween(($currentObject/Expiration), [%CurrentDateTime%])+0.5))
In addition to the approaches Tim suggested, you could used the following:
toString(round(hoursBetween(($currentObject/Expiration), [%CurrentDateTime%])))
this will return an integer formatted as a string
if you want a certain number of decimal places, you can use this syntax
toString(round(hoursBetween(($currentObject/Expiration), [%CurrentDateTime%]),1)
this expression will return a number with one decimal place formatted as a string