Well, my advice is to start a program and keep going until your son resists/loses interest. You will definately know how much is too much, your son will tell you (though perhaps not in as many words). Just as you knew when he was full while he was nursing/taking a bottle, you'll know when he's had enough time sitting down and doing lessons.
Right now BeanBean (3) is willing to sit for up to an hour and a half doing some things, but for others lessons can be no longer than 15-20 minutes. It's all good. I'm not anywhere near the point where I feel like I'm forcing him to work, but he is being challenged and working hard. He looks forward to doing lessons (spending time with Mamma and learning something new? How much better could life get!) and enjoys them, as well as discussing the new things that he's learned with his grandparents, cousins, daddy, etc. It's all in good fun.
I'm trying to save some money to get him the Singapore workbooks.. but it's looking like it won't happen for a while.
I've been making stuff up for him based on the textbooks, but I'm afraid that it's much more boring coming from Mamma (there's no color, for one thing