Well, it all depends on the individual - how far does one have to travel each day? How many hours a week would one work (at least 10 if it's part-time)? Would one work through the week or on the weekends? How much time would one devote to homework (revision and/or assessments) each week? Does one have to work or would mum and dad offer to cover the relevant costs? How much of a social life would one like to have? The list of questions can (and does) go on.
Keeping in mind that most students work (14 hours a week on average, I believe), I personally found it easy to manage 12 hours a week of part-time work (shifts at night, spread over the week), 3-4 hours a day of travel time and a first year class load of 24 to 22 hours a week. It would be best to say that I didn't exactly push myself in academic terms and that I was happy enough with a single night out a week, but I knew of a number of fellow students who were quite happy and successfully managed to work, learn and play till the early hours of the morning, but they were all fortunate enough to be living close to both the unviersity and their place of employment.
After all of that, I suggest that you take it easy in terms of part-time work if you have the chance, but I think that our little stories are pointless given that the only way you'll be able to work out just what you can manage will be by working it out for yourself as time goes by. Only you know what you are capable of and what you would like to achieve, after all.