lock!!! seriously though, how long is a piece of string, depends on distances ridden. i carry everything except kitchen sink on all rides.
i carry:-
2 tubes
puncture repair kit
3 metal tyre levers
alien multi tool inc chain tool
15mm spanner for fixie nuts (when on fixie)
brompton specific tools when on brommie (pliers, adjustable spanner)
waterproofs (top, bottom, overshoes, over mittens, hat)
pump
zip ties
lights inc lion battery pack
energy gels (just in case)
spare folding tyre
gloves
sunglasses
mobile phone
gps
food and drink, how much, dependent on ride length.
fleece for stops or breakdowns, thin for summer, thick for winter.
all in my carradice camper on a rear rack.
all learnt from experience of when things go wrong on long rides far from home.
hope that helps.