It depends on the location, frequency, and intensity of the bumps. If it was before the start/finish it could easily be 5/10ths.
Last night I was running lemons junkers on the same track. I chose an older FR car while others were driving newer, faster FF cars. In their draft, I could gain 1.2 seconds over my best qualifying time...assuming the other cars battled it out and allowed me to stay in the draft. When I had the lead, one of them bumped me down the straight a few times I was still 0.7 seconds quicker...but I might have been able to improve that if I knew exactly how much speed to scrub off for turn 1. I tend to aire on the side of caution when someone gives me a boost.
The benefits of someone bumping you can be exponential in cars with stock gearing. In my case, 3rd gear was very tall for exiting the final turn but 2nd was too short. One or two bumps exiting the corner was all my car needed to get back into the power band.