Maximum cooking time for beef joint in slow cooker?

I usually cook a joints of beef by browing it in a pan and then putting it in the slow cooker all day (about 8 hours) completely submerged in stock.

At the weekend I have family coming over, so the joint I have got is twice the size of the usual, and we are eating a couple of hours earlier than usual.

As I do not really fancy getting up in the early hours of the morning to put the beef on, I am thinking about putting it on last thing at night. This would be a cooking time of around 15 hours.

Is that too much? Is there a maximum time that beef joints (or any meat, for that matter) can be cooked in the slow cooker?

Would I be better to cook it the day before and then reheat it?