Based on an online dictionary :
Inside or not further than an area or period of time
And here are some examples (provided by the dictionary) of how 在…里 or 在…内 is used :
Two thirds of Californians live within 15 miles of the coast.
In 1992 cross-border controls within the EU were dismantled.
For orders within the UK, please enclose £2.50 for post and packing.
We recommend that this wine should be consumed within six months.
Within hours of the tragedy happening, an emergency rescue team had been assembled.
From those examples, we can see that "在" is being omitted in some sentences. Now, what I want to ask is :
Are we allowed to omit 在 anytime we want to or is there any specific rules/conditions of when we are allowed to omit 在?