Canada decided to delay second doses of the three main vaccines in use here: Pfizer, Moderna, and AstraZeneca. The motivation was to get more first doses into people. The math there was
100 people get first doses, about 70% protection -- 70 protected people
50 people get both doses, about 95% protection -- 48 protected people.
Since 70 is more than 48, that was the public health choice made.
Now what you're asking is, when it comes time for second dose, will some people now get 90% instead of 95% because it was delayed?
The early studies on second doses did them really quickly. 4 weeks, even 3. But there were other studies with longer gaps and there was real world data with longer gaps where for various reasons people waited 12 weeks or 16 weeks. While many expected waiting would make it worse, that's not what happened. Waiting improved protection. People were more protected 12 weeks after their first shot (without a second) than 3 or 4 weeks after it. Then yes, it goes up even further when they get that second shot. (Quotes later in this answer.) They seem to show that 12 weeks is the "sweet spot" for a second dose, but the truth is from a public health point of view, it's not about maximizing one individual's protection to 96 or 97 rather than 95, but about the overall population protection.
Here in Canada we were originally scheduled for second doses 16 weeks after firsts, but it is now changing to 12. This is not so much because you get better protection at 12 than at 16 as it is that Delta does much better on singly-vaccinated people than the other variants do, so it's important to get a lot of people onto second shots before Delta takes hold here as it has in the UK.
This CBC news article quotes some experts:
"There is no question that for the whole of Canada, from the perspective of lives saved, that giving single doses to people and asking them to defer their second dose was the best idea," said Dr. Allison McGeer, a medical microbiologist and infectious diseases specialist at Toronto's Mount Sinai Hospital.
A new Canadian preprint study, which has not yet been peer reviewed, found the Pfizer-BioNTech and Moderna vaccines' effectiveness grew from 48 per cent 14 to 21 days after the first dose to 71 per cent after 35 to 41 days.
So, I have no numbers about Sputnik's effectiveness after 12 weeks vs after 16. I am just showing you that the decision isn't based on that. It's based on vaccine supply, and the effectiveness of a single shot against the variants that are currently circulating. If you think that you should get a second shot now rather than someone else getting their first, taking two shares from a limited supply, let me give you quote from Dr. McGeer from that article:
"Of course I want my two doses of vaccine. But if the price of that is somebody else maybe dying because of it — that's just not OK. I think we can all see that."