There is no payment schedule as such - we can see from the US statements of account published by the WHO here, and available (for the US) back to 2012, that payments over the last 8 years have been made at fairly random intervals, and never before the payment's due date. The last payments made by the US were in January 2020, roughly a third of which went towards paying off the remainder of their 2018 dues, the remaining amount being put towards paying off their 2019 dues.
The outstanding balance currently for the US sits at a total of around 100 million USD plus 100 million CHF, roughly 40% of which is owed as assessed contributions for their 2019 membership.
The longest gap between two previous payments in the available data was between December 12th 2016 and October 13th 2017, a gap of 305 days. The second-longest was between March 13th 2013 and December 26th 2013, a gap of 295 days. The data is shown in the boxplot below.
Given that at the time of posting (May 22nd), we are only at 136 days since the US's last payment (indicated by the red dot), this is so far not out of the ordinary for the country, although it is a larger gap than the median.
This is, however, clearly not the international norm. The UK, for example, has made all of its payments in full over the last 4 years, with a clear schedule; usually around 3-4 weeks before the payment is due.