Should I send my 18 months daughter to a half-day or a full-day pre-school/enrichment centre or should I wait till she's older before putting her in for such long hours?

It depends on your schedule. If you are working and there is no caretaker to look after your child if she attends a half-day program, then you only can place her in full day. Ideally, if I had flexible working options and work from home, I would choose to place my child in a half-day program so I can use the rest of the day to build up on what she learnt in school and spend the rest of the day bonding. If you have no choice but to place her in full day, then do go for a trial class to see if your daughter likes the place. Also take extra notice of the teachers and how they interact with the children. It'll be even better if you are able to observe the school from outside on a normal working day.