Normal business hours are the same in Thailand as in most other countries. It's still common for many companies to work a half day on Saturdays. However, it is highly unusual for Thais to request a business meeting on Saturdays. In general, Thais do not worry as much about being on time as Americans or most other westerners.
Thais do not take long annual vacations, as westerners do. Most of them will not take more than a day or two off per year. However, there are a number (about 15 days in all) of public holidays on which almost all offices will be closed. You should be aware of upcoming holidays and avoid scheduling your visit around these times. Note that many of the holidays are based on the lunar calendar, so their dates will change from year to year.
Although annual leaves are uncommon, there are still some times of year that are best avoided. The worst time to visit is the first half of April. Chakri Day falls on 6 April, and then from 13 to 15 April is the Thai New Year (Songkran) Holiday. With weekends, everything comes to a complete halt for up to five days. Many Bangkok residents escape the city for the provinces, leaving the capital practically empty. The first week in May should also be avoided, as both the 1st and 5th are holidays.
Meetings, Appointments and Traffic: Although traffic in Bangkok is not quite as bad as it was ten years ago, the time it takes to get from one place to another can still be unpredictable. Generally, being up to a half hour late for an appointment is not unusual, nor cause for alarm. Bangkok has no real central business district where most companies are located. In fact, the trend in recent years has been for many large companies to move to more suburban locations. Given this situation, it is almost unheard of to be able to fit more than two meetings in one day, unless the third is over dinner. Appointments will usually be made at 10:00 in the morning or 2:00 in the afternoon. Breakfast meetings are unheard of, in part because most Thais don't eat breakfast as well as the problem of sending the kids to school. Many office workers with kids have to send their kids to school, often making them a little late for work.