My ds is 5 months old, and absolutely HAS to have that last catnap between 5 and 6 p.m. He is one grumpy bugger if he doesn't get that in. He is also not a good napper, although the past few days have been better. He has usually been a 45 minute napper, with me struggling to get him to extend his naps. The past few days (knock on wood) he has started taking anywhere between 1 hr 15 min and 1 hr 45 min naps. Hallelujah!
(I'm sure I say this, and tomorrow will be a nap day from he!!).
I usually just lay my ds down in his crib around 5 p.m., and he'll take a quick 30 to 45 minute nap. If we have to be out and about, he will usually fall asleep in his carseat. But if he doesn't get that nap, he is such a grumpy butt. But that catnap seems to get him through until bedtime.
I will say this for ds's naps, and I'm by no means any sort of expert, but maybe it will help. First, as he got closer to five months, I absolutely have to keep him awake for 2 hours between naps. He may act sleepy earlier than that, but if I put him down, it is a 45 minute nap, and nothing longer. Second, I used to let ds catnap between 5 and 6 p.m. in my arms, but that proved to be bad news as he got a little older. He started fighting the catnap. But once I started putting him in his crib, he would go straight to sleep.
Maybe that helps a bit! Good luck!