My spouse has dementia. Do I need to file for a guardian or conservatorship?

It depends on the circumstances and the degree to which the spouse is mentally impaired. If your spouse executed a properly crafted durable power of attorney and health care proxy giving you the authority to make financial and healthcare decisions you may be able to delay, and possibly completely avoid, seeking court intervention. Note that a failing spouse cannot execute a durable power of attorney or health care proxy if he or she is legally incompetent, in which case court intervention will be required. Also note that simply because someone has been diagnosed with dementia does not mean that they are legally incompetent.