The textbooks will tell you “by 18 months”. However it does depend a bit on what method of locomotion a child is using before he/she gets up and walks. 15% of children never crawl. They may do a sort of commando crawl or shuffle on their bottoms (I used to do that apparently and so did my eldest son) or roll everywhere and these children often walk later than 18 months, sometimes beyond their 2nd birthday.
Occasionally delayed walking (beyond 18 months) is a sign of a more serious developmental problem and it is probably worth getting your child checked out by your GP or paediatrician. It is not necessarily linked to delay in any other aspect of development. www.babycentre.co.uk/baby/development/walking has a nice explanation of child development leading to walking written by health professionals but the site also has lots of advertising which I do not necessarily endorse. There is a wide range of normal when it comes to stages and ages of development and remember babies born before 37 weeks of gestation (ie. more than 3 weeks early) are allowed to be behind your sister’s/friend’s/neighbour’s child born at full term at the same time….