For targets running subbuilds from an orchestrator, .ci.yaml allows control the orchestrator builder's execution timeout with field timeout.

There is no way to control the timeout of the subbuild. The subbuild drone builder is shared across all targets, and is pre-defined from infra/config side:
https://flutter.googlesource.com/infra/+/refs/heads/main/config/engine_config.star#95
which uses the default timeout.MEDIUM (60min).

One concern with subbuild timeout change is: it will affect all subbuilds though we may be only interested to control one subbuild of one target.