@Cybro- they will not be stampped based on the number of days the actual Visa is valid for. So, do not worry if they will be stampped for just 18 days. Even if their Tourist Visa will expire in 18 days from the time they enter the country, they will be stampped for 3 months, 4 months, 6 months or for any number of days that the POE officer deems suitable (with the maximum of 6 months at a time)