Plenty of time. It’s not a hard question to answer, is it?
Did the Russian State authorise this attack? Or was it made possible by the fact that the Russian State has lost control of the weapon?
Depends what your goal is. If you want some phrase to blow into a yet another international scandal - yes, the amount of time is about right. But that will be all you will achieve. Plus it will help those actually responsible to get away.
If you actually want to find out what happened and arrest those people - it's much harder. Such an effort would then require lengthy joint investigation to find everything out about the origin of that substance. But in this political climate nobody will go for that - nobody in the UK will want to put their career at risk by a routine matter-related trip to Russia or a meeting with a Russian investigator.
A success example - work with Argentinian authorities on the heroin channel. They've been working together for 1.5 years until they discovered everyone who was involved into the "operation". And nobody screamed in media that Russian state is involved into drug trafficking somehow, and was giving 36 hour windows. Appropriate professionals from both countries were just doing their work, until the involved were held accountable.