Well, my almost 6 month old ds was never a great napper to begin with. Consistent 45 minute naps all the time, but still cranky as anything! We had some limited success in extending his nap times to an hour, or maybe 1 hr 15 min. Occasionally he would sleep for 2 hours or so.
When he learned how to roll over both ways, he would disturb himself even more, so the 45 minute naps went down to about 20 or 30 minutes! But after about a week of this, he has actually started sleeping better and longer during his naps. (BTW - his nighttime sleep has been great for months).
I switched Ethan over to a 4 hour EASY schedule at about 5 months. He just wasn't finishing his feeds at 3 hours anymore. He was definitely ready to go to 4 hours. Now he finishes his feeds most of the time (although he doesn't always want the entire bottle at his first feed of the day ~ usually around 7 am, not sure why that is, but whatever :roll: ).
As for activity time, I absolutely have to keep him up for 2 hours between wake time and nap time. He will start getting cranky sometimes at about 1.5 hours, but if I put him down there is no way he sleeps longer than 45 minutes. But 2 hours or so, and he's down.
I will say this, now that he's rolling. I put him down on his back, and he sleeps for about 30 minutes. Then, he wakes up, and I can hear him flip over, and move around, and babble to himself. He sometimes stays awake 30 to 45 minutes doing this! And then he'll finally fall asleep. I absolutely refuse to go into his room unless he starts out and out wailing. Because he does do his little mantra cry sometimes, but he's just getting "comfortable", that's what I think. If I go in, it makes it worse for him!
Hang in there, it will get better. Keep us posted on your progress!