That depends on what you require for your monthly expenses and living.
Calculate backwards to your risk-reward appetite to come up with the amount of money you will need for the size of your trading account.
As an investment and a full time Forex Trader, you will of course need to have sufficient capital with...this is the most important part...a good, well regulated, and trusted Forex Broker.
For starters, do not go with ICMarkets a.k.a I-See-Bucketshop because, even though they are ASIC regulated to give you that sense of false security, is well known to manipulate their client's trading account and you will land up here (and other sites) filing a complaint on being scammed by a scam broker.