Hello Miwa,
I discussed the signal with NFC technical team, It's very hard to give an accurate continuous time on "High" state, because at the time PN5180 is handling data, and you know , data size determines the of "High" state, so from datasheet, the "time" was not listed.
After sending data to PN5180, you will have to wait until the line's state to be LOW. So in software, you had better use "interrupt" operation for BUSY signal.
Hope above advice can help you!
Best Regards,
weidong