Q: How long is a criminal trespass good for in Texas?

A: All law enforcement needs for an arrest is "probable cause" to believe you committed a crime. However, in order to win a conviction against you, they'll either need you to enter a plea (like guilty or no contest) or they'll need to prove each and every element of the offense beyond a reasonable doubt.

They charged you with the offense of Criminal Trespass. Showing that you violated a criminal trespass warning that was issued four years ago isn't enough to convict you because there are additional elements in the definition of the offense, and that gives you an opportunity to defend yourself. A person commits this offense if they enter or remain on or in property of another, including residential land, agricultural land, a recreational vehicle park, a building, a general residential operation operating as a residential treatment center, or an aircraft or other vehicle, without effective consent and the person: (1) had notice that the entry was forbidden; or (2) received notice to depart but failed to do so. See section 30.05 of the Texas Penal Code.

So, in the situation you describe, there are at least two easy arguments to start with... First, this isn't even really "property of another" because you have basically bought it from the owner and are in the process of paying it off. Maybe you don't have a written mortgage agreement like many people but any attorney should remember from their time in law school that, as problematic as it may be, an oral contract is still a contract. You were living in the home and paying under that purchase agreement... so it's your home. The second argument is the "effective consent" element of the offense. It may be true that four years ago you didn't have consent, but in the years since you not only have effective consent but you also have ACTUAL CONSENT which can be shown by your purchase agreement and any other documentation that can be used to show that the person who owns the home "on paper" not only is aware that you live there but actively approves of it and is collecting payments from you because you live there.

If everything is as you say then a decent criminal defense attorney could probably get this case dismissed. It might be fast or it might not, and that usually depends on how stupid or lazy the prosecutor is plus how much internal political pressure they are under to protect the stupid law enforcement officer who made this decision.

Unfortunately, it is expensive to hire a decent criminal defense attorney and you won't get that money back if you win. You could take your chances with a cheap or court-appointed attorney but that is risky because those guys want to resolve cases as quickly as possible to avoid losing money on the case... and the quickest way to resolve a case is to convince the defendant to take a plea and to not "rock the boat" by calling out stupidity or laziness on the part of prosecutors and law enforcement.

Bottom line is that the old criminal trespass warning gives them enough cover to arrest you without opening themselves up to a civil lawsuit for their incompetence but it probably won't be enough to convict you if you can afford a proper defense.