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?

Start with half day cc first for at least 1 month then gradually convert to full day cc. This is a more gentle transition for a new environment. If you are not working or have other family member to look after your child at home, half day cc is sufficient as most of the curriculum are executed in the morning. Afternoon schedule are mostly nap, snack and light activities. In addition, it is good for the child to bond with yourself/grandparents at home.