iPhone/iPad battery charging/discharging cycle

Apple recommends at least one charge cycle (charge to 100% and then running it down completely) per month.

Then, is it OK to charge iPad/iPhone all the time as long as I have charge cycle per month? Or, do I have to refrain from recharging iPad/iPhone until it's completely discharged?

The full discharge/charge cycle is mostly to recalibrate the charge controller and the time remaining indicator. Since the battery doesn't change that much in one month, you could very likely get by leaving it on the charger all the time except for only a few full cycles per year, until it starts getting old and changes more rapidly.

You're probably fine charging whenever - a monthly charge cycle or two is more "a good idea" and "recommended" than "required".