Word meaning not knowing when to stop

It does rather depend on context. Reckless will often fit, but so might unrestrained, undisciplined, careless, and several others.

Personally I think the best "single-word" answer is intemperate - having or showing a lack of self-control; immoderate. It's particularly associated with not knowing when to stop drinking alcohol, but that doesn't prevent its use in other contexts.

Note that temper has many meanings; in "intemperate" it's the verb to moderate, or tone down.

Depending on context, you may use unrestrained or unbridled.

Compulsive might do what you want.

Some possibilities: blithe, sense 2, "Indifferent, careless, showing a lack of concern"; uncurbed, "Unlimited; unrestricted"; gung ho, "Very enthusiastic or energetic"; high-handed, "Using power or authority without proper consideration for the feelings or rights of others"; and perhaps manic, speed demon, overboard, over the top, heavy-handed, lead-footed, roughshod, heedless.

Some facetious suggestions: unbrakeable; brakeless; brake-challenged.