Hi Wendy,
It started taking Kieran about 10mins to fall asleep about this age. It used to be much quicker (as long as I had the right A time). I'd literally put him in his cot and off he would go. I found that right around 6.5mos, he started taking some extra time in his cot to settle himself. During our naptime routine, he would push away from me when we were having a cuddle and he just didn't want to be held. So I started putting him down earlier and letting him settle himself. I just adjusted our nap time routine and made sure I put him in his cot about 10mins earlier than I wanted him asleep. He usually takes at least 5mins and up to 10mins. If it starts taking 20mins then I know I have his A time wrong. If he is happy and rolling, babbling it is usually UT. If fussy and mantra crying, usually a few mins too late and slightly OT.
HTH!