a 5x5 program can be done for as much as a year. you can cycle them over and over even if they are written to be a 4, 6, 8 14 or 16.386 week program. its not like the body building myth filled zines that are only trying to get you to pay out a lot of money for rehashed crap that pros dont even do. plus the pros are the pros for a reason. decades of work that no one else would do. decades of eating that no one else would do. decades of drugs and supplements that no one else would do. so why only do part of what they do.
the reason you dont see much isolation work is that its not needed. 5x5 is not a bodybuilding plan. bodybuilding can benefit from a lot of volume. after many many years each body part starts to require soooo much volume that is where isolation workouts came from. a pro would need so much volume to produce growth that it would also take about a week to recover. the other 99.99999% of the worlds population does not need that much volume and therefore can recover much quicker, say in a few days. 5x5 is a strength based program. yes a bigger muscle is a stronger muscle but 5x5 doesnt just focus on bigger. it focuses on better. and here better is stronger and bigger. its a different type of training for a different type of goal.
isolation can be added, but IMO it should only be added to fix weak links. i like to think of not that i am doing more for a weak link but less for the strong links. they are already strong and therefore do not need the same volume.
5x5, or what some see as linear progression can be done for 3 month, 6 months, or even a year or more. depends on the goals of the lifter, the training age, the nutrition, the sleep and stress habits of the lifter. you will commonly see a switch to a 3x5 program which allows for more supplementary work more similar to what you are used to in the idea of a split but that supplementary work is only meant to bring up the main lift of each day. if it doesnt bring up the main lift then it doesnt matter. so the supplementary work can change every 3 month, 6 months, or maybe just once a year. with that supplementary work always changing that means a structured 3x5 can be done for years. sure, there may be blocks of 4 weeks or 8 or 10 weeks. but the plan can be cycled again for years.
and speaking of blocks that i just mention, that is a main theme in periodization. which is something i dont usually see in body building programs. the block has a goal, or a wave. then another block is done. you can do block after block after block to accomplish a long term goal. this is why these plans can go for years as each block is planned which allows you to plan better for the next block.
does that make sense?