  We present the combined results of three photometric monitoring campaigns
targeting very low mass (VLM) stars and brown dwarfs in the young open cluster
IC4665 (age ~40 Myr). In all three runs, we observe ~100 cluster members,
allowing us for the first time to put limits on the evolution of spots and
magnetic activity in fully convective objects on timescales of a few years. For
20 objects covering masses from 0.05 to 0.5 Msol we detect a periodic flux
modulation, indicating the presence of magnetic spots co-rotating with the
objects. The detection rate of photometric periods (~20%) is significantly
lower than in solar-mass stars at the same age, which points to a mass
dependence in the spot properties. With two exceptions, none of the objects
exhibit variability and thus spot activity in more than one season. This is
contrary to what is seen in solar-mass stars and indicates that spot
configurations capable of producing photometric modulations occur relatively
rarely and are transient in VLM objects. The rotation periods derived in this
paper range from 3 to 30h, arguing for a lack of slow rotators among VLM
objects. The periods fit into a rotational evolution scenario with pre-main
sequence contraction and moderate (40-50%) angular momentum losses due to wind
braking. By combining our findings with literature results, we identify two
regimes of rotational and magnetic properties, called C- and I-sequence. Main
properties on the C-sequence are fast rotation, weak wind braking, Halpha
emission, and saturated activity levels, while the I-sequence is characterised
by slow rotation, strong wind braking, no Halpha emission, and linear
activity-rotation relationship. Rotation rate and stellar mass are the primary
parameters that determine in which regime an object is found. (abridged)
