I've developed a hernia at the old stoma site, quite soon after surgery. It hasn't gotten any worse for me in the last few months, so I'm putting off surgery. Plus it's only 12months since I had my major op's.
As long as it doesn't bother you or isn't worsening then you can manage it, otherwise they can operate to close the opening. I believe they use a mesh insert to reinforce the weak spot across the muscle wall.