OK this is the route I've taken, but I'm sure there are other (probably better) ways of doing this.
For my 35mm rocket I wanted to take a picture every two seconds for one minute, that way I'd get 30 photos which is plenty to fill up a film.
I used 555 timer chips to do the timing, if you're not familiar with the 555 it functions in different ways depending how you connect it up. One is the monostable, it uses a 'trigger' switch and once it is triggered the output remains on for a set amount of time. You can change the amount of time by using different value resistors, there's a formula so you can work out precise values. Another mode of the 555 is an astable, this has no trigger but what it does is switch the output on for X amount of time then off for Y amount of time, again you can determine the values.
What I did for my camera controller is use an monostable and an astable. The monostable had a g-switch for the trigger and was set up so it stayed on for 60 seconds. I wired the output of this chip to the reset pin of the astable. For the 555 it will only work if the reset pin is receiving power, if there is nothing going to the reset it won't function. My astable timer was set up so it went on for one second then off for another.
When the rocket lifts off the switch is triggered, this turns the output of the monostable high, the reset of the astable receives power so the astable starts to do its thing. This is linked to a relay so the relay clicks on and off taking pictures. Once the sixty seconds is up the monostable turns off which makes the astable stop and no more photos are taken.
That was pretty long winded explanation, this website probably explains 555 timers better and gives you schematics for mono/astables.
https://www.eleinmec.com/article.asp?4
One little note about modifying cameras, they contain capacitors which remain charged even after the battery is removed and can give electric shocks so take care. I learnt the hard way.
