Too frequent is too much and too late is too late. Don’t harass your subscribers with emails, do not follow them every hour with an email asking them to buy your products.
Often, toning down frequency if you are a regular mailer improves response rates. Don’t send newsletters in an untimely manner like super late or super early - like 10pm at night or 3am the morning, this could cause them to not take your emails into consideration in the appropriate times. Consequently, they might simply forget about your offer.