How long does it usually take for VIP status to be approved after I've written reviews?

...approved and posted it should be about a day. Sometimes it's less, possibly a bit more. The usual time frame TER gives for most everything is 72 hours. But I've had most things be much quicker than that.

The thing is you don't get the 15 days for your very first review until you submit a second review and it's approved and posted. Then you get 30 days, 15 for each. From then on you get 15 days as soon as any review you submit is approved and posted.

Check out the Self-Help Center on the upper right of most discussion boards right under the red and white life preserver. Just click on the link there. It will answer a lot of the questions you may have about TER and how it operates.

I have a similar question about reviews: what happens after you the first 30 days, the 15 thereafter, but lets say you cannot do a review every 2 weeks because you cannot see a provider but once a month?

If you want continuous VIP, you will have to pay for it under what you describe.  Any days you earn from reviews or problem reports will be added on to what ever you have paid for.  The best deal is to pay for a year at a time.

Swim

...to specifically answer your question though, what happens after the first 30 days runs out is your VIP membership ends and you don't have access to the VIP privileges of the site ( you are still a basic member though) until you submit another review and it is approved and posted. So you either write a review every two weeks or you pay for the VIP membership to keep it continuous.

A year is the lowest cost per month but I've paid for a 90 day membership and by posting reviews and doing a few problem reports I've been VIP for more than 6 months. FYI: The monthly membership automatically renews and your card is charged every month. You have to cancel it to get it to stop. The 90 day and yearly do not auto renew. If I don't submit a review in the next couple of weeks I'll have to pay for another 90 day. But it's worth it to me to do that.
There is a Pay Cash option if you don't want a paper trail.