The FAA knowledge test is 60 multiple-choice questions in 2 hours and you need a score of 70. It is a memory and interpretation exercise — weather products, airspace, regulations, performance charts, aeromedical factors, weight and balance. It is entirely beatable with a ground school course and a fortnight of honest study. Under 14 CFR 61.39 the result must have been obtained within the 24 calendar months preceding the month you take the practical test, and that expiry is your real deadline for finishing the certificate.
The checkride is a different animal. A designated pilot examiner spends roughly 90 minutes to two hours questioning you on the ground, then an hour and a half in the aircraft watching you fly to the tolerances in the Airman Certification Standards. Most of the ground portion is scenario-based. Not what the regulation says — what you would do on a particular Tuesday, in a particular airplane, with a particular forecast and a passenger who wants to get home.
A disapproval is not rare and it is not a catastrophe. You retrain the task you missed, collect a fresh endorsement under 14 CFR 61.49, and retest — usually within a fortnight, and usually only on the item you failed. It never comes up again.