How can I configure more space around horizontal line in frac?

I don't think this is a good example. If you use it inline the result will be awful; see the following example.

\documentclass{report}
\begin{document}
Here is some text with inside it a fraction; I add some text
before the fraction so it will appear in the middle of the
paragraph. Probably we do now and the fraction is
$\frac{\mbox{Time}}{\mbox{h}}$
hereby shown. Extra text is added after the fraction in
order to show how amazingly awful the result is.
\end{document}

The problem doesn't show when you properly display the fraction:On the other hand, if the fraction appears displayed, again
after some text for getting at least two lines
\[
\frac{\mbox{Time}}{\mbox{h}}
\]
the result will not show the problem, which is due to the
fact that \TeX{} expects script size material when typesetting
an inline fraction.

If you want the fraction inline, use the slashed form together with \text and amsmath:

 \usepackage{amsmath}

A text fraction can be now

 $a+\text{Time}/\text{h}=42$

while in displays you can still say

\frac{\text{Time}}{\text{h}}

But using “word fractions” is not very good style, in my opinion. Using a properly defined symbol will give the fraction its real mathematical sense.

Usually the size decreases in the numerator and denominator, but \mbox prevents this. \text from package amstext or amsmath (the latter loads the former):

\documentclass{report}
\usepackage{amstext}% or amsmath
\usepackage{lipsum}
\begin{document}
    $\frac{\mbox{Time}}{\mbox{h}}$ vs.
    $\frac{\text{Time}}{\text{h}}$
\end{document}