I think it's completely normal to have second thoughts . I did for awhile after I ordered mine. I was scared I would find some thing I liked more while I was waiting for mine to be made. I haven't and still like my first choice best:)
6-12 months beforehand if possible ! This means you will be able to choose any dress and have it in time:) my dress has been 4 1/2 months so far and could be up to 7.