Hi, if you send your team on Training Camp it only boosts your chances for the next match, no longer effect. The only way it could have a longer effect is that if you used TC and won the match due to which the morale of your players increased and caused them to perform better in future (I seriously doubt anyone thinks that way when using training camp) so it's just about that next match, nothing more.
How significant would this advantage be? It impacts the match results a lot in my opinion. For instance, recently I had a cup match against the 2nd position team in the league ( Me being the 1st position team) I used training camp while my opponent did not and I won like 4-0, two days later I have a league match with that same opponent and according to the spy my opponent planned to use the same tactics as before and we would think if I have countered these tactics before with great success (4-0) then I'll beat him again but I did not, instead I lost 5-1 this time, why? Because he used training camp for that match and I didn't. This was just an example, of course he could have made some tactical changes in the things like pressure, tempo etc. which spy doesn't inform of or it could just have been luck but most likely the reason for the result of both of these matches was the training camp.
About the 25% advantage, I don't know how exactly the game engine decides the result of the match but lets say it just boosts your chances a lot more than they were before. (25% is a good lot because it's not like you had 0% chances before.)
How much time training camp takes? It is instant, no time taken, even if you pressed the training camp button only 1 minute before your match starts the effect would be there and it will be activated.
You can use like 6 training camps in one season, first one costing 10 boss coins, using it a second time costs 20 boss coins then 50 then 100 then 200 I think and so on, I'm not sure how much boss coins the 5th or 6th training camp uses but you get the idea.
If you use Training Camp your opponent receives a notification warning that you are using it so if you don't want them to know about your training camp before the match then first use Secret Training followed by a Training Camp.