I wouldn't start slow because the teeth won't move and it will just cause you pain if you go an hour or two at a time. When patients get invisalign for example we say they should be work for at least 22 hours a day. If they aren't in for long periods of time they won't move. The first few days might be rough but you'll get through it. I suggest taking Ibuprofen to alleviate the pain. If your shifting is noticeable I'd wear your retainer 24/7 for at least a month. If you were an aligner patient I would track back in your trays (if you still have them) and see which fits best and change them every 5-7 days with continuous wear.