From what I’ve seen (and from chatting with friends), Discord tends to roll out free Nitro trials sporadically, often based on engagement things like active usage, server participation, or gaps since your last trial.
I got one about a year ago during a reactivation wave for dormant users, then another this February when I logged in after a long break.
The fact it showed up on PC but not mobile suggests it’s also platform-targeted.
So don’t expect a regular schedule, just pop-ups when Discord deems you “worthy” of a nudge.