I've done it (well, Plymouth to Santander, actually).That crossing is 20 hours and I believe those from Portsmouth are a little bit longer.
I should also say that it was horrendous (and I went in July). I do not suffer "mal de mere" but I came quite close in mid Biscay. It cost me around £400 return (with a car) and I then had to drive across to Barcelona and face an eight hour crossing to Palma Mallorca. Fortunately the Med was a little kinder to us.
I had to travel that way as I was carting some stuff to a friend's apartment. I would not do it again.