Should a disabled congressman be allowed to serve indefinitely?

As U.S. Senator Tim Johnson (D-SD) recovers over the next number of months, the question becomes pertinent.

The Twenty-Fifth Amendment to the federal constitution is clear: a disabled president can be involuntarily withdrawn from the office. That Wikipedia backgrounder even writes of an event where Ronald Reagan risked removal from office, due to "perceived laziness and ineptitude."

Try to disregard the precarious balance of power in the Senate, because the answer to this question will resound long after the present matter is resolved. Together with the House of Representatives, the Senate passes legislation, declares war, impeaches presidents, regulates currency, and establishes rules of immigration. A single senator can filibuster most of these actions and effectively kill them. Given this, should a months-long disability not disqualify a U.S. senator or representative from service?

And how long of a disability is too long?