I understand that waiting for the journal to get back to you is a stressful and nailbiting time, more so since you want your paper to be published by a certain deadline. The time it takes for a journal to get the review process completed varies across journals and fields. While some take a month or two, others can take up to 6 months or more. As you have correctly observed, Statistics/Mathematics journals usually take a longer time, and may at times, take even up to a year to complete. But do not get stressed.
First of all, check the journal's website for information regarding expected review timelines. If you fail to find this or you think their timelines are too extended for you to get your paper accepted by the deadline you have in mind, I think you should contact the journal editor. Explain your situation to him or her in detail and request for an update.
You will find a template for writing to the editor for a status update here: How long should I wait for a response from the journal? You can customize it to suit your request. The editor is likely to respond to this email. If however there is no response for over 10 days, you can write a follow-up email as well.
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