From the documentation we have:
pwm_frequency = clock / ((2 ^ exp) * divide)
where clock can be 1kHz, 32kHz or 19.2MHz, exponent can be 0-7 and divide can be 1, 3, 5 or 6
As such the minimum frequency should be 1000 / (2^7 * 6), which looks to be a
minimum duty cycle of 760ms. The pwm size can be specified as either 6 or 9
bits, which if I interpret the documentation correctly defines the period.
So I think the longest period you can set is 393 seconds (512 * 1/freq), with
duty being a multiple of 760ms. So I believe you should be able to find some
configuration inbetween that suits your timing needs.