S46 controller providing OJT experienced a loss of separation event when failing to take timely control of a developing conflict; the reporter indicating more seasoning time is needed before requiring newly certified controllers to conduct OJT.

Narrative

An E120 was inbound from the west at 5000; heading 070 for wake turbulence spacing. A CRJ was inbound from the east on a heading of 270 at 5000. The 070 heading and the 270 heading were well more than the 3 miles required for in trail separation. The trainee turned the westbound CRJ to a heading of 250; and then told the eastbound E120 that their vector would take them across final. I immediately put a J ring on the westbound CRJ because I felt that the vector; in conjunction with the vector across final; might be too close; but there wasn't anything I felt I could do. An earlier turn to the E120 would have caused a wake turbulence error with the aircraft in front; and a turn to the north for the CRJ wouldn't have happened fast enough to save separation. By the time the E120 was told to turn right to a heading of 190 it was too late; and separation had been lost. Recommendation; there should be a time/year requirement for OJTI's (on job training instructor) on certain positions in the TRACON. There are times that I don't feel that I can recognize and correct a trainee's mistake in a timely manner to keep myself from having an operational error. How far can you let a trainee go and still be an effective OJTI? My comfort level might be 4 MIT (miles-in-trail) but the trainee is trying for 3 MIT; do I make him/her work to my standards or not? This is my first RADAR facility. Just a few months after becoming fully CPC I received a trainee on departure; and then after he/she checked out I received another trainee on departures. The whole time I never got quality time on the arrival wall. After my second trainee finished on departure; I got a new trainee on arrivals. My honest and frank suggestion is to remove me from OJTI for a few years to get the proper experience needed to train on final.