We can think about periods of time or points in time. When we think about periods, we are thinking about a length of time. A length of time has duration. A period of time can always be used as the answer to a how long question. Here are some examples of periods of time:
- twenty-five years
- two centuries
- five years
- three and a half hours
- hours and hours
- five seconds
All of the period phrases above can be used as the answer to the question How long did it last?.
We don't use since with periods of time. If we need to use a preposition in a phrase or sentence like the Original Poster's, we use the word for :
- Serving science for 25 years
- We have been serving science for 25 years.
We use since with points in time. A point in time can often be used as an answer to a when question. Phrases representing points in time sometimes appear with the word ago. Here are some periods in time:
- last year
- three months ago
Notice that we can use the phrases above as answers to the question When did it happen?
We use since with points in time:
- since Tuesday/ last year/ three months ago/ 1984