There is no good or definite amount to start with. Invest as much as you can afford to loose as forex is very volatile market.I would not limit my self with the time on demo account. You have to demo trade until you are making consistent profit. Then I would go for a cent account with 25 and then 250 and the 2500. You start each step after you have gained consistent profit on the previous one. Dont be in a hurry to give your money away. remember that 95% of traders lose.