Only you know when the time is right. Quite a few single parents have rules such as not bringing a man/woman around their child until after they've been dating for few months or things have long-term potential. No one wants their child to get attached to people only to have them rooted out of their life time and time again. Some parents worry about how dating several people will effect their children's opinion of them.
yes, being single parent is already a hard situation for a kid to be understood. as well as for the kids seeing different person around their house. as for me it is better to introduce someone only if I am sure of such decisions.