Hello,
I've never used contactless, as I've had Oyster cards since before the contactless started being accepted. But the general theme is very similar.
re: journey durations, these vary depending on the zones you travel through and how you travel. To avoid problems when train spotting I have sometimes left a station and then either within a few minutes re-entered it or touched-in on a bus (without travelling!) and then re-entered the station.
The reason for such seemingly bizarre actions is that some stations are what are known as 'out of station' interchanges (OSI) and at these you can exit the fares paid area and then a short while later re-enter the fares paid area with the system then treating this as a single journey which is charged a single through fare.
This is great if you want to change trains, say between mainline and underground at a major London terminus, but not so good if you want to reset the clock on your journey duration.
Below are some links which will explain the situation more fully and also detail the OSI station pairs / groups.
http://www.oyster-rail.org.uk/interchanging-trains/
http://www.oyster-rail.org.uk/osi-list/
http://www.oyster-rail.org.uk/maximum-journey-times/
Also, when passing through ticket gates allow at least a second (if not two) between the person in front of you removing their card from the yellow card reader touch pad and you placing your card on the card reader touch pad. This is because sometimes when people go through too closely to each other the gates may still open even though the data exchange transaction with the second person's card has not been successful. This scenario especially arises when many passengers are touching out. I've had this happen to me a few times but I've always spotted it and with the assistance of a member of staff station been able to resolve it.
Although card reads take just milliseconds, when there are hordes of passengers (especially commuters) trying to pass through the gates as quickly as possible it is still possible to be 'too quick' for the system!
Enjoy your visit
Simon