There is no monthly contribution requirement to have a Roth IRA. Furthermore, the amount you put into a Roth IRA per month is less important than the amount you put in per year. Since your goal is to let the time-value of money work in your favor, it can be an advantage to make a one-time annual payment.
For example, put your tax refund or an employer bonus into your Roth IRA and not make other contributions during the year. That allows you that contribution to benefit from compounding faster and for longer than if you contributed monthly.
However, for many investors, monthly contributions to a Roth IRA are an excellent way to add discipline to their investing goals.