When to follow up for recommendation letter
I'd wait until about 5-7 days before the due date, and then send a polite email reminding them that the letters are due, and offering to have the links to the forms resent if they need it.

There is a balance between being "pushy", and sending them a reminder too late for them to avoid feeling time pressure if they haven't written it.  I try to stay on top of things, but if I forget, I would prefer to receive notification more than a day or two before the due date.

Of course, if there's a situation such as "rolling admissions", where an on-time submission can still be a bad thing, I'd ping them earlier --- but a situation such as that should never come up, as you would have let them know that an early letter was really necessary at the time you made the original requests.

It might be a good habit, when sending them the "thank you for agreeing to write me a letter" message, to say something like "unless you ask me not too, I'll send out a reminder about a week before the due date".