I had an 18 month old who cried for the first 3 weeks off and on for hours when I put her down to nap in the playpen. I don't think she ever actually slept. The first few days I would leave her for about 40 min, then an hour, then one and half hours, then 2 hours. After 3 weeks she started just walking up to her playpen after lunch and would go right to sleep for 2 hours. I guess some kids just take longer to understand that you mean business and they have to stay where you put them and they're not getting out until nap time is over so they might as well accept it and sleep!