How far apart should my years be if I'm doing a 25 year timeline and have 10.75 inches?

Get a ruler in your hands. Measure things until you start to understand how a ruler works. Measure some stuff and figure out where the center is. Say you measure a book and it's 7/8" thick. You look at your ruler and see that every eighth is divided into two sixteenths, so obviously half of 7/8" is going to be 7/16". If you write that out you have 1/2 x 7/8 = 7/16. And you notice that 1/2 is divided into 2/4 and then into 4/8 and so on, so you can convert anything to anything by multiplying all the numbers on top and then all the numbers on bottom.

Other rulers are divided into 10 and 100 parts. But an inch is still an inch, so anything on one ruler can be translated to the other ruler. A half inch on one ruler is 5/10 or 50/100 on the other. An eighth inch is just 12.5 marks when you have 100 marks per inch. A metric ruler divides an inch into 25.4 parts, so a half inch would be 12.7 of those parts. Pretty simple, isn't it? Practice this a bit and people will think you went to wizard school.

10.75/25 = ? Now this is a trick question. You want 25 marks, but that means only 24 spaces between marks. So the answer is 10.75/24 = 0.4479 inch. Since you can't measure that accurately, you might want to reduce your graphic to 10.32 inches wide and then the marks will be 0.43 inch apart.

Here is another trick: if the timeline starts at year 0 then you have 26 marks and 25 spaces between them. If you put the marks 0.41 inch apart they will take up 10.66 inches. I will leave it to you to figure out exactly what you want to do.