I have a mulberry tree growing in a large pot, about 30-40 litres (about 40-50cm diameter, same depth). How long will it be fine in that size pot, or is it good forever and will just be stunted?
To some extent, it depends which variety you've bought - Morus alba 'pendula' is a smaller, neater version of this plant, and the largest known specimen of this particular variety is 4 x 4 metres. Even that, though, would require a larger pot as it grew than the one you currently have, probably something of the order of 90 high by 60 cm wide.
If, though, you have any other variety, ultimate height and spread after 20 years in the ground is 9 metres high with a spread of 10 metres. Clearly, that is not achievable in anything other than a very, very large pot, in particular if you wanted to use the fruit. If its stunted, it won't fruit very well.
Note that this plant tends to grow rapidly at first, (maybe the first 4 years or so) and then is slow growing thereafter. It also has roots which are fleshy and easy to break, so potting on needs to be done extremely carefully in order not to damage, or expose to the air, the roots - it dislikes the disturbance. For that reason, it is not a plant which is good for pot culture, and without knowing which variety you have, its impossible to hazard a good guess how long it will be 'fine' in that pot size, but I'll take a leap in the dark and suggest a couple of years at most.