I've been practicing playing blues harmonica for over a year now (with some breaks of a few weeks)
I'm noticing that after about 20-30 minutes of practice time my lips and mouth become tired and my single notes become sloppy.

I use puckering / lip blocking to get single notes.
I can bend the the holes 1,2,3, 4 and 6. 

If I exercise on the bends seperately and slowly, I have full control over them and I can keep my mouth relaxed.
When I play a (fast) song/riff and have to use bends, my mouth is a bit more tight and I lack control over the bends, often I position the harmonica a bit upwards to achieve the bends.

Is 20-30 minutes too fast to get tired from practicing harmonica?
If so, how should I keep myself from tensioning my mouth during hard parts of riffs and difficult bends?

I'm aware of the adviced pucker embouchure (no small opening with tensed lips, but a duck-like wide seal with the harmonica) and I'm holding my harmonica deep enough between my lips, but some tension is needed to get the single note (bends), isn't it?