Opponents of a plan designed to guide development at Lake Tahoe for the next two decades will have until early next week to file a legal challenge.Most of the elements of the Tahoe Regional Planning Agency's Regional Plan Update take effect today, agency spokesman Jeff Cowen said. Opponents have until midnight Tuesday to file a legal challenge.Proponents of the Regional Plan Update, including many from the business community, have lauded the plan as a way to bring needed investment to the Lake Tahoe Basin while still protecting its environment.The most vocal opponents of the plan, have been environmental groups that …