How much time would I need to commit if I join the reservists as a infrastructure engineer at 18?

If you are looking to join the Reserves and go to university research (just google it and quite a lot comes up!) University Officer Training Corps (UOTC). It's like Reserves but for uni students, you get paid (slightly more than a Pte soldier as an Officer Cadet (OCdt), everything is geared around uni (ie most UOTCs have their training on a Weds eve (ie sports afternoon)), and on weekends you're always finished at mid-day on a Sunday. Summer camp is during uni hols and then you might have other opportunities to do army stuff (unit attachments, adventure training exercises etc) also in the summer hols. AND . . . there is absolutely no commitment whatsoever to actually join the army or any part of the military! It's a fantastic stepping stone to both the Army Reserves and the Regulars but there's no real push towards either. As a previous Ocdt (3.5yrs doing my degree and then quit a PGCE) and then as Directing Staff at a UOTC i absolutely cannot recommend it enough!!! Beats working in a bar, or working in a shop etc because the opportunities you get are awesome!! I've told my daughter if she goes to uni, just join the UOTC!!! To get about £100 'working' a weekend when it's just total fun with your friends is a brill wage!!! There are a certain number of UOTCs around the country but every Uni is affiliated to one -some require more travelling than others (your travel is paid by the UOTC). For example - if you're at Reading Uni they travel to Oxford UOTC, Plymouth Uni travel to Exeter UOTC, Portsmouth uni travel to Southampton UOTC etc etc.

If however, you look at joining the Reserves (if you don't go to uni or decide the UOTC isn't for you) then minimum is up to you - most units would require you to do either 19 days (a National unit) or 27 days for a local unit.
Good luck in whatever you decide to do!