I had botox for TMJ spasm two weeks go. Lot of improvement. But will its effects continue gradually or is this as good as it might get?

Probably maximum. At 3 weeks, you're probably at maximum improvement. You may need regular injections every 3-4 months. Occassionally, the need for the injections decreases over time, likely because the underlying disorder has become quiescent.
Maybe. Tmj is caused by strain on the joint, so an injection alone will help but this must be combined with continued antiinflammatory medications and a strict soft diet for several weeks to achieve the best results. If you are grinding your teeth at night, a bite block from your dentist may also be necessary. There is no quick fix for tmj!
Might but. At the end Botox treatment is not permanent , instead try to treat the underlying reason for the TMJ problems ..
Botox tmj. Botox is used to treat muscle spasms that can cause headaches and jaw pain. This does not address the underlying issue of a malocclusion ( bad bite) . So the relief wlll be temporary. Find a dentist /orthodontist that can thoroughly evaluate you- including the possibility of sleep apnea.