Good Q!

There really isn't a standard amount of time needed between relationships, imo. It depends upon the depth of our previous relationship and how it ended. It has taken me years to get over some relationships. I was over others before they even ended!

I would therefore recommend looking at the current relationship closely. What exactly are the factors that's gotten you (or friend, if that who you're posting for) interested in this new person? Is it physical, emotional, or some other factor/s? Are the factors similar to the person that you (or they) were previously involved with? If so, then a step back would be a healthy thing to do. It isn't a matter of time. it is a matter of recognizing that this new person is different, there is a spark of interest that doesn't remind you of your ex. You don't have recurrent thoughts of your ex, especially ones that bring up dark emotions inside.

That's my perspective anyway.