When To Stop My FWB Relationship Due to Potential New Partner

My ex of 8 yrs and I recently started a friends with benefits relationship. We have been having sex for a couple of months (6). I have met someone who i will potentially date. We are still getting to know each other and he has not asked me out yet but I anticipate he will. However I still want to have sex with my ex bc I trust him sexually. I would not be having sex with my new date for a while (until I know him very well). When should I end my FWB relationship? What should I say if my new potential partner asks the last time I've had sex and with who? 😐