simply subtract them and apply a format of [h]:mm.
If you need this number to perform calculations (ie you mutliply hours by x cost), perform the operation and then multipy by 24 and format as general
Beware that without including the date, the operation will fail if your time range passes midnight. You can buy yourself 1 free pass of midnight by using
=B1+(B1 < A1)-A1
or
=(B1+(B1 < A1)-A1)*24