According to experts there is no such thing as "the right time". It depends on so many things. From finacial issues to psychological ones. And then some say that if you wait till your first one is 12 mnths then will be difficult cause he is growing up and he needs all the attention he can get, wait till he is 18-24 mnths and he goes to childcare/kindergarten and again he will need you. But then if they are close ages it's good for them cause they can study together and help eachother when need. and if they are so far apart as age, the older one can be like an idol/good example for the young one.
I think whenever you feel ready and know for sure that you can handle two, then go ahead and make yourself happy.