It depends on many factors such as the type of reflector and the amount of ventilation you have, but plan on at least a foot, probably more. Watch for any leaf crisping and move the light further away or use a fan. A flower in particular will be sensitive to the heat.

I'm curious if the light is just being used for a single rose plant? That light is suitable for lighting at least ten square feet, probably twice that depending on what you wish to grow.