Hi there, I can understand why you're a little confused. Each baby's signs are different, my DS also yawns at around 20 minutes but I know he's not sleepy yet because the other signs aren't there as well. For my DS, his eyes get heavy and his body becomes more still. I also know that an hour is the longest he can go in the morning. Later in the day, the yawns become my main cues and once I get two of those within 5-10 mins, I know it's diaper/story/sleep sack time.
I wouldn't recommend keeping your LO up for more than an hour at this young age. My LO is 3 months now and I'm slowly transitioning him from 1.15 to 1.5 hrs. At 6 weeks, an hour is plenty! I hope this helps.
Cheers,