When is a reasonable amount of time for patients to be on 90 mg BID of ticagrelor before transitioning to 60 mg BID following PCI?

That depends on the type of PCI.

Generally speaking, I do not continue Ticagrelor beyond six months for stable angina PCI and one year for PCI in the setting of acute coronary syndrome.

If the patient has had recurrent ACS or complex bifurcation PCI, I would consider long term dual anti-platelet therapy. I prefer to switch to clopidrogel 75 mg daily after one year.

Various factors determine dose and duration. Generally speaking, plaque rupture events treated with DES PCI require 12 months of DAPT, while non-ruptured plaque events can do with 6 months. High bleeding risk patients can have a truncated DAPT course of 1-3 months in some circumstances.