at least
数量 + は is used to express a minimum amount, meaning 'at least' that number.
Use 数量 + は to emphasize that the stated quantity is the minimum, not the maximum. It carries a nuance of 'at least this, if not more.' You often see it when setting conditions, making requests, or expressing expectations.
This structure is common in everyday speech and writing. It's not formal or bookish, so you can use it in conversations. The meaning is close to 'at least,' but it's specifically about quantity, not time or degree (though time is a quantity). For example, when you say '三十分はかかる' you mean it will take at least 30 minutes, implying it might take longer.