[Jodie.]

It seems the length depends on how the previous series rates so starting with BB14:

-BB12 & 13’s ratings were disappointing for C5 at the time, so BB14 was 68 days
-BB14 rated 0.3m higher than the previous, so BB15 was 72 days, the longest C5 series
-Then after BB15 decreased from BB14 by 0.3m, they reduced BB16’s length to 66 days
-After the lowest ratings ever at the time for BB16, 1.4m, BB17 was made the shortest series ever at 50 days
-However after an improvement in that series with it rating 1.6m again, BB18 was increased to 54 days

But because BB18 is now the lowest rated series ever were probably looking at about 42 days lol