How training should someone have before they are deemed competent for a specific task?

Deeming someone competent needs to be against objective criteria. There is no fixed time period. For some tasks it may take just a few hours, others months. You need to state what does the laboratory needs them to be able to do. For a particular task start by naming the task and documenting the training and competency requirements.

For example 
1. Task: Weekly maintenance of instrument x. 
2. Training requirements: Witness task being demonstrated and was supervised for three operations. 
3. Competence criteria: The instrument operator must be able follow the maintenance work instruction x, completing maintenance steps one to three and thereafter perform the instrument qualification test, step four.  
4. Criteria for evaluation:  4.1 The laboratory manager (or other authorised person) must approve training record by reviewing and signing. 4.2 Witness the operator perform the task, meeting the stated competency criteria.

Now for a specific Operator, record evidence of supervision and training. When completed, the laboratory manager should declare and record the observation during witnessing against the criteria. For example “the operator was witnessed to follow the work instruction. The instrument qualification test performed (ref 2020/02/09, attached to training records) passed as per established limits. Operator “M” is therefore declared competent to perform task x. Competence will be monitored by his supervisor through witnessing and record review".

As personnel training and competency is a critical activity, the Advisera ISO 17025 toolkit includes the mandatory procedure as ISO 17025 document template: Competence, Training and Awareness Procedure along with 4 appendices: Training Program, Training Record and Performance Monitoring, Record of Attendance and Competence Approval and Authorization Record. You can preview the template at https://advisera.com/17025academy/documentation/competence-training-and-awareness-procedure/