If I was common law spouse, married from 1990 until this very day and my spouse died, would I be entitled to his social security?

It depends on whether you have a valid common law marriage or not. You do not indicate in which state where you live. NC and GA does not recognize common law marriage. PA did recognize it up until 2005. They have since abolished it. You indicate that you were "married" in 1990. In the states where common law marriage is or was recognized, each state has different requirements for whether a valid common law marriage exists. Each case is fact specific and must be compared to the facts in the decisions which have held what is or what is not enough. So you would have to consult a family law attorney who practices in the county/state where you reside to ascertain whether a valid common law marriage exists. If the marriage occurred in a different state in which you now live, then you need to talk to a lawyer who practices in the state where the marriage occurred. If a valid common law marriage exists then the Social Security Administration would recognize this.