the small ones not long typically in game each spell can be cast so long as he has enough focus, the in game resource. focus is returned to the casters each turn which translates to about 10 seconds on the battlefield. As for the feats these are typically big f-off spells that signify the immense power of the warcaster(which is what these people are called) and can only ever be used once per battle so I believe it would take at the very least a day before he could summon ghosts again.