Trading in altcoins can be started even at 0.001btc but that kind of trading doesnt worth any think,the minimum amount i say should be 0.25btc reserved for that.Its not neccessary to use them all at once,must that much must be in reserve.
I think even 0.1 btc would be sufficient.In fact there is no minimum amount needed for trading but if you want to make big,invest big but you should be able to afford the risk also
0.25 is actually just too much for a newcomers to trading job,0.001 is quite effective if you can manage to always making profit and the fees doesn't matter at this time,otherwise still better to save money and gain more to start with higher balance but there's nothing wrong to start with 0.001