After a testing proposal has been published, third parties have 45 days to submit "scientifically valid information and studies that address the relevant substance and hazard endpoint, relating to the testing proposal" (REACH, Article 40 (2)).
A table on ECHAS`website presents the substances, hazard endpoints, deadlines for submitting information and links to the submission format for which ECHA is currently requesting "scientifically valid information" from third parties.

Any scientifically valid information and studies received from third parties will be taken into account by ECHA in preparing its decision. ECHA publishes its responses to these contributions for the testing proposals after adoption of the related final decision. Responses will indicate ECHA's conclusions drawn from the information provided by third parties. All confidential information has been removed. By doing that, ECHA wishes to improve the transparency of the process and encourage submission of relevant information.

More information: http://echa.europa.eu/consultations/