you don’t have to wait for your I-751 to be processed you can file n-400 while your I-751 is processing, if you do so you should know that your I-751 along with your n-400 will be transferred to your local USCIS office and you also should be prepared for combo interview, there’s no timeline, after filing n-400 some cases were processed faster and some cases were delayed it all depends on your local office. good luck