there is no average.
My eldest child slept 4 to 5 hours at a time for the first few weeks and was settled into a 7 hr stretch at night by 6 weeks.
My 2nd child slept in a similar pattern from day one also, except he HAD to have a nap at 10am every day and would be screaming if he was not in his cot by 10.15. This was regardless of what time he woke up in the morning and if he didnt wake until gone 9am he was not up for long pmsl.
Number 3 baby was born, had a bottle, and by the time he was 1hr old had gone to sleep whereupon he slept for 12hrs!! He slept in 5 to 7 hr stretches from the day he was born and by the time he was 3 months old was sleeping an 8+hr stretch at night.
On the other hand my next door neightbour had a baby who slept for 1 hr at a time from birth until 3yrs of age!!!