I live in a rented apartment in HongKong and work 6 nights. The only time i get during week days is after 10 AM, that too if the weather permits.
How often shall I take my child out?
Is it that the kids who stay indoor too much become introverts?
I don't think there is any connection to indoors/outdoors and introvert/extrovert.
However, a 18 month old is really developing very fast, and should be developing social, motor and language skills.
it is not clear from your question if your child is cared for in a nursery/daycare or if he/she is at home with you while you work? Do you mean going 'out' to playdates, or outside for walks?
If the child stays inside in the same place all the time, I would worry about stimulation. Does he/she see enough new things? Does she get to experience different things, and hear a lot of words?
If you kids is indoors all day, but have plenty of social interaction, I would worry some about stimulation too - seeing cars, clouds, dogs, people on bikes or what else offers a lot of opportunity of language building and general discovery about the world
In addition, being with other kids helps with social skills - at 18 months they should have begun learning not to pull on other kids hair (lots of babies are hair pullers), they might need interaction to learn not to push/bite. They can also begin learning to share, though true sharing comes later.
This doesn't need to be all the time, plenty of parents have their kids at home, but if you don't see anyone else in his/her age group I would think a playdate a week would be great.
Gross motor skills
Going back to being indoors all day, I would mostly about physical development - gross motor skills. Does he/she get to walk and run and climb on things?
Climbing, beginning running should be going on. Going out doors helps with gross motor skills, but also just physical activity in general. If the child does not move a lot, he might not be developing the muscle tone he/she needs to.
I don't think a specific amount is really definable, in particular because each child is different. However, the AAP (American Academy of Pediatrics) endorses "Caring for our Children", a set of national standards for childcare environments (ie, daycares).
Infants (birth to twelve months of age) should be taken outside two to three times per day, as tolerated. There is no recommended duration of infants’ outdoor play.
Toddlers (twelve months to three years) and preschoolers (three to six years) should be allowed sixty to ninety total minutes of outdoor play. These outdoor times can be curtailed somewhat during adverse weather conditions in which children may still play safely outdoors for shorter periods, but should increase the time of indoor activity, so the total amount of exercise should remain the same;
That sounds about right to me as a minimum; my kids need around an hour to two hours to be happy. But really you should construct your outside time around various external and internal factors, including: