This depends on your dog and what you’re training. For general obedience, training is most effective when broken up into multiple small 5-20 minute sessions throughout the day. I always suggest reading the dog to make sure they are motivated enough and enjoying the process. Once you see them losing interest, increase their interest in what they are working for and go out on a high note. This is especially true with most obedience behaviors, we want our dog to look at training like we look at our favorite hobby not like homework. Aim for at least 30 minutes daily, broken up into smaller sessions. As long as your dog is motivated and enjoying it though, you can train as much as you want.