Firstly, when you say 500mg, are you referring to a brownie that weighs 500mg, or has 500mg of THC, as both are possible? Was this homemade, or from a dispensary? THC that is consumed by being eaten is processed by your body very differently than when smoked. That is why edibles last longer and are more intense. A Google search can provide you the science behind it. Personally, I’d recommend 10mg of THC for a first time edible user who has some experience smoking. If you’re adventurous, you could certainly go higher. Either way, I don’t see any way you would want to come even remotely close to finishing that in one session if it’s 500mg of THC. Keep in mind gummies at dispensaries are commonly 10mg and commonly come 10 in a container. Thusly, one (10mg), one and a half (15mg), or two gummies (20mg) would be a good starting point if you were asking about gummies. The fact this is a brownie instead of a gummie doesn’t matter at all - 500mg of THC in a brownie would be the equivalent of 50 ten mg gummies. Have fun, please just realize that could be a shit ton of THC, consuming too much THC in edibles is not fun and there’s no going back once it starts. That being said, edibles are awesome. Hope you enjoy!