Our REST API Responses time out after 30 seconds.
Unless written within the subscription contract, there is not a commitment for Marketo to provide an SLA for REST API Responses.
As mentioned in our API License document: http://developers.marketo.com/api-license/ in section 6 B:
"B. No Support. Unless otherwise agreed to in writing, Marketo shall not be responsible for providing any support, maintenance, or other services (or level of service) to You or a user of the Software Application for the Marketo APIs."