The most recent official data on average length of family-related leave taken by parents is from the Maternity and Paternity Rights Survey in 2009, which collected data from parents of children born in 2008 across Great Britain.
This shows that in 2008, 23% of employed mothers took their full statutory entitlement of 52 weeks of maternity leave or more (e.g. by taking annual leave in addition to Statutory Maternity Leave). According to the Labour Force Survey, in 2008 there were 350,000 employed mothers with a child under one.
The full Maternity and Paternity Rights Survey 2009/10 Research Report can be found here: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/214367/rrep777.pdf . Information on the length of maternity leave is available in Tables 2.11 and 2.12.
We are currently commissioning a new survey which will provide updated information on family related leave and pay entitlements including Maternity Leave. Subject to the progress of data collection, we anticipate publishing findings in Summer 2019.