As you've (re)discovered, the cases
environment doesn't automatically line-wrap long explanations. As a remedy, I suggest you encase the contents of the cases
environment in a custom array
environment which allows line-wrapping. Note that it's not necessary to employ \text
directives for the explanatory textual material.
[For some reason, I can't upload a screenshot of the output that's produced by the following code. I will try again later, i.e., in a few hours.]
A general comment: I think your readers would strongly benefit from you finding a more succinct way of stating the clauses "if $j$ gives the very high return and has the slope of the curve inward towards other peak points" and "if $j$ gives the very high return and has the slope of the curve outward towards other peak points", especially since these clauses differ in only one word: "inward" in the first clause and "outward" in the second.
\documentclass[a4paper, 10pt, conference]{IEEEconf}
\usepackage{amsmath} % for 'cases' environment
\usepackage{array} % for '\newcolumntype' macro
\newcolumntype{P}[1]{>{\raggedright\arraybackslash}p{#1}}
\newenvironment{myarray}{%
\begin{array}{@{} l P{0.5\columnwidth} @{}}%
}{%
\end{array}}
\begin{document}
\begin{equation}\label{eq:eq:mylabel}
J_i(G)=
\begin{cases}
\begin{myarray}
+\infty, & if $j$ gives the very high return and has the slope of the curve inward towards other peak points; \\
0, & if $j$ gives the very high return and has the slope of the curve outward towards other peak points;\\
-\infty, & otherwise.
\end{myarray}
\end{cases}
\end{equation}
\end{document}