Whilst Pedro's answer is strictly correct, it doesn't really Peter in his quest for more realistic trackwork. Ignoring the different types of chairs, which might be specific to a particular time and railway company, (various types of which can be found in the C&L and Exactoscale ranges) in general the normal spacing for transverse sleepers on plain track was around 2' 6" or a convenient 10 mm in 4 mm, centre to centre. This spacing would be closer at each rail joint, perhaps only 2' 0", and often a wider sleeper was used (12" as opposed to 10" elsewhere). Wooden sleepers were, before WW1 or thereabouts, 9' 0" long, later reducing to 8' 6". When working with OO gauge it is better to consider the "overhang" rather than the actual length, so a OO gauge sleeper would be either 33.5 mm or 31.5 mm long, to take into account the 2.5 mm (I know 2.33 mm) narrowing.
According to the current Railway Group Standards available on the Internet, new construction, using Continuous Welded Rail, should have sleepers at no more than 700 mm centres on straight track, reducing to 650 mm elsewhere. (8.5 mm in 4 mm). The document, dated October 2003, also states "Jointed track shall have sleepers spaced at not more than nominal 760 mm centres" (Back to the 2' 6" of yore.)
Sleeper spacing for pointwork is another matter, and is another can of worms, but it is likely that the spacing would normally be around 2' 0" but a number of parameters have to be taken in to account, including the track geometry and the need for maintenance, as well as railway company preferences.