Why is gastric lavage considered to be helpful only if a patient comes to a hospital within 1 hour of a toxic ingestion? How severe would the effects be if a patient came to the hospital after the recommended 1 hour time frame?
Gastric lavage only washes out ("lavages") the stomach ("gastrium"). Gastric emptying time is generally ~60-120 min, probably on the lower end of that for ingested liquids. After that amount of time, most of the toxic substance will be beyond the level of the pyloric sphincter (i.e. in the duodenum) and not accessible to lavage.
The second question is not possible to answer without some information about what specific toxic ingestion you are interested in knowing about.
Gastric emptying, which used to be well-accepted and seems intuitively beneficial, should not be routinely done. It does not clearly reduce overall morbidity or mortality and has risks. Gastric emptying is considered if it can be done within 1 h of a life-threatening ingestion. However, many poisonings manifest too late, and whether a poisoning is life threatening is not always clear. Thus, gastric emptying is seldom indicated and, if a caustic substance has been ingested, is contraindicated (see Caustic Ingestion).
If gastric emptying is used, gastric lavage is the preferred method. Gastric lavage may cause complications such as epistaxis, aspiration, or, rarely, oropharyngeal or esophageal injury. Syrup of ipecac has unpredictable effects, often causes prolonged vomiting, and may not remove substantial amounts of poison from the stomach. Syrup of ipecac may be warranted if the ingested agent is highly toxic and transport time to the emergency department is unusually long, but this is uncommon in the US.