Assuming I can gather 500$ a week from here and there,how much exactly should I invest.
I'm not that greedy and there is no time constraint.
Just holding bitcoin is enough to make your $500 grow 25% in months time only or even faster and larger than that. Theres another feature that you can relax and rely on the time also just holding it from a secured wallet like blockchain.info,mycelium or for more safety measures trezor wallet or ledger Nano.
I guess there is no percentage measurement that we should follow in trading unless you have a knowledge on how to trade your money. However, holding your coin also as a investment will also grow but if you really wanted go have some experience on how to trade then I guess just put only a small percentage of your earned coin.