2 grams your first time is a good rule of thumb. Most people can handle more, but all drugs affect different people in different ways.
I have a naturally high tolerance to most drugs, but I'm a lightweight when it comes to mushies. I've only done them five times, 2.5 still gets me tripping properly, and they were brf grown so not as potent as they could have been. I ate a handful of fresh puerto ricans once and drank some rum and got fucked up. I haven't ever had a 'bad' trip, but that was a difficult experience.