I leave mine together until I want to fatten up the ewes in early August. Lambs get de-balled anyway so it's not an issue. If like us you only have a few acres you may find this easier as long as there is enough grass to fatten up the ewes in time for tupping. I've done this for over 20 years with a small flock, never had a case of mastitis and never needed to buy creep feed. Frankly, I would never keep my ewes from getting overweight if I took their lambs away. The ewes walk away after a few seconds of suckling from an older lamb anyway, they'll do their own weaning.
Without knowing the age and condition of your ewes, how much grass you have and what quality it is and a bunch of other factors I wouldn't presume to give you advice based on my own experience to be honest, just wanted to say that under some conditions you can leave them together.
Use your best judgement and if something seems to make sense, it probably does.