How long would it take for a horses mouth to go hard?

[Fools Motto]

Raced once as a 2yr
Back into work at 4 - light hacking.
Now in training for polo, done chukkas for 6 months, played first game recently. Meanwhile hacking/cantering to keep fit.
Been mouthy/leaning on forehand for about a month.

Am I right in thinking, the 'hard mouth' could have happened anytime during the above, and now it could be just teeth issue?

And could someone kindly explain their definition of a hard mouth please. Just in case I'm thinking wrong.

Thanks.

 

[deicinmerlyn]

Why a hard mouth because he's leaning?!
It sounds like a training issue, bitting, or, pain either in the mouth or elsewhere.