Sleep

I think i need to get my 4 month old on a better sleep schedule, well i say schedule but i know things dont always work out that way!  Were blessed as She s pretty much always been an amazing sleeper but she kinda sleeps on our schedule at the mo - goes to bed with us at 10 then i wake her up at 9ish) i feel like she needs to start going to bed earlier as 10pm is too late to continue with as she grows up surely. My question i guess how long would you leave your little one to sleep in a row? At the moment she is going for an 11 hour stretch- should i put her to bed at say 8 and then set an alarm to wake her up 11 hours later or do you guys leave your little ones to sleep longer?? Ive never left her for longer than that but im sure she would go longer but how long is too long? Dont want to be setting my alarm to silly o clock just for my peace of mind if she doesnt actually need to be woken up if you see what i mean but i always worry as she is only on the second centile, 10lb 5 oz at 4 months! Docs dont seem concerned though as she is diddy in height too and has always followed the same line consistently from birth? Sorry im going on now arent i! Thanks in advance x