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?

If your daughter has never been in a childcare or infant care I suggest to start her off half day so length of separation from you is not too much for her to handle. By the time she is coping well in half day then you can increase the hours by putting her in a full day programme. That way she will not be too overwhelmed and it also gives you time to adapt to a new schedule.