Yep, it will depend on each individual - i guess you can just go by other peoples experience and see what suits you....

DD#1-DD#2 - 29 Months - perfect, great, love the age gap. They are now 5 and nearly 3 and are best of friends.
DD#2- DD#3 - 19 months. Harder, good coz both still have day sleeps at same time. But looking back at photos, my Dd#2 really was so little when i had DD#3....i worry if she missed out on anytihng.....
DD#3 - baby #4 - 20 months. Will let you know in about 20 weeks. I again worry coz i think DD#3 is still such a baby, she not walking yet, she is very much a mummys girl.
In all honesty, i loved the 29month age gap. But age is not on our side (me-33, DH-36) and i guess we wanted our babykingdom days over sooner rather than later...

Good LUck !