How long to make an ultimate skin?

My one friend doesn't understand why it takes so long to make an ultimate skin and i assume its actually much harder than we think it is, also i believe you do it to make it some what of a special thing? like an event without...the event? it's cooler to have the wait and try and guess or think of what you guys at riot could be making for an ultimate skin, but yeah if a riot member on the skins team could give me a little detail on the process that would be cool!