Wikipedia:Silence and consensus
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Consensus can be assumed to exist until voiced disagreement becomes evident (typically through reverting or editing). You find out whether your edit has consensus when you try to build on it.
In wiki-editing, it is difficult to get positive affirmation for your edits. (Disaffirmation comes with a revert.) No matter how many people on a talk page say they support an edit; it is only when your changes are reverted or substantially changed that you learn that you did not, in fact, have full consensus.
Of course, it is impractical to wait forever for affirmation: in the meantime then, you can assume that silence implies consensus. You can continue to hold that assumption (hopefully safely) until someone comes along and changes the page by editing or reverting. The more visible the statement, and the longer it stands unchallenged, the stronger the implication of consensus is.
Most of the time, you will find that it's fine to assume consensus, even if just for now, as it's more important to keep editing and cooperating smoothly in good faith as much as possible.
When real people are affected by a decision, such as blocking users, or using material covered by the biography of living persons policy, positive confirmation is preferred. Even in these cases, however, dissent might show up later, and it is then no longer appropriate to assume consensus.
Wikipedia is huge and our editors' time is limited. At any given time, there are many open discussions on many different topics across the project. We encourage our editors to be bold and it is highly likely that you will eventually find yourself affected by the outcome of some decision that you didn't know about, or didn't have the chance to join. Where a decision is based mostly on silence, it is especially important to remember that consensus can change.