The two main things to bear in mind are your costs and being competitive in the marketplace.
First, add up all of your costs for the year, including any mortgage, utilities, cleaning, maintenance, advertising, insurance etc, so you know what income you need to make a profit. Then scour holiday let websites for similar-sized properties in your area to work out how much they typically charge at different times. Weekends will command a premium over weekdays if you’re happy to accept stays of less than a week.
You don’t want to be the cheapest, or the most expensive, but it’s better to start off higher and reduce rates if bookings aren’t coming through. If your property happens to be near a popular annual event you can charge a premium as accommodation will be in high demand.