I cant believe some of the replies in this tread. Not only are people interjecting with suggestions that the champions league isn't that hard because they won it with a big team once, but were having suggestions ranging from about 8-15 years.
Personally I would say at least 15, probably more like 25-40.
Here are the problems I can see.
Getting a few fairly quick promotions in the lower leagues is possible, but eventually you get to a position where you find yourself in a league where your reputation and stadium size is very restrictive. If your a historically small team from near the bottom of the BSN/S you wont suddenly start to attract much more than a few thousand fans due to your stadium size and reputation. To expand you stadium costs money, plus your reputation wont increase as fast as your stature for example if you take Gateshead to the championship and you have a twenty thousand seater stadium you will still only attract 5-8 thousand fans mainly helped by away support. This is realistic Sunderland and Newcastle fans from the local area wont suddenly start supporting Gateshead because they've made it to the championship.
Having to pay to expand your stadium and not getting big gates results in you not being able to compete in the market for the standard of players you need.
Another problem you have with the reputation lagging behind is that players wont be attracted to your club as you have a low reputation.
This makes it hard to take a BSN/S team to the premiership. Not to mention the fact unambitious boards can force you to sell players, not let you expand your stadium or improve training facilities when needed etc.
As for the champions league. Securing a squad good enough to challenge for fourth in the prem takes time you need a couple of world class players and you cant just get these on free transfers, you have to bring through youth and slowly improve your squad in the transfer market, which takes ages! Once you've qualified for the champions league, you then need to win it, which is a challenge and takes a combination of luck and skill even if you are at a top club.
Remember not cheating is abiding by these guidelines
The traditional definition for LLM in the CM/FM series is:
Starting with sunday league experience
Managing in as low a league as possible
Not using any exploits to generate money unrealistically
Not using any cheat programs Genie, FMRTE etc
Using your own (non exploitative) tactics
Not taking tips on players from other people
No Reunion players, or similar
Plus you should probably be playing with attribute masking on, otherwise it is too easy to search for players and click around teams just looking at their players regardless of how obscure they are.