Hi there! To calculate how long it would take to earn enough for 0.9 BTC, we need to know the current price of BTC. As of my last update, the price of BTC is around $35,000. So, 0.9 BTC would cost approximately $31,500.

If you earn $500 a week and save $250, you would be saving 50% of your earnings. To earn $31,500, you would need to work for approximately 63 weeks (31,500 / 500 = 63).

However, this calculation assumes that the price of BTC remains constant. In reality, the price of BTC can fluctuate greatly, so it's important to keep that in mind when making financial plans.