Journey times from Waterloo to Twickenham vary between 20 and 30 minutes, depending upon which train you catch.
The fastest ones (20 minutes) run every half hour (usually at 20 and 50 minutes past the hour), with Wokingham as their final destination.
Additional half-hourly services (taking 23 minutes) run at 28 and 58 minutes past the hour, with Windsor & Eton Riverside as their destination.
Trains at 3 and 33 minutes past the hour follow a circular route back to Waterloo, taking 30 minutes to get to Twickenham.
The times above are based upon lunchtime services for this Saturday. For other times, check here:
http://www.nationalrail.co.uk/
From Euston to Waterloo you'll need a southbound Northern Line train but you need to take care because the Northern Line splits into two, with only one route serving Waterloo. The train you require will probably be shown as 'via Charing Cross'.
Chris