How difficult is the Latin of Seneca considered to be? I have just discovered him in translation and am wondering where in ones studies it is reasonable to take a look at his writings.
Depends what experience of Latin.you have. I assume you mean Seneca the Younger. His style is rhetorical and pointed, and may take a bit of getting used to, but his syntax is generally straightforward and not very complex, and his vocabulary is unexceptional for the most part. Why not give hm a try?
thank you!
As a postscript to MWH I think you need to be careful what you read. In prose the letters might be the best place to start. His style takes some getting used to and he is not easy to translate as he is not verbose and his thought can be quite compressed. Grammatically as Michael says it’s pretty straightforward.
The plays are of course magnificent. Start with Thyestes.
It may change your life! 
Speaking as one who has taken Seneca’s advice on this and “taken the plunge”, I can definitely recommend Thyestes. As far as grammar and vocabulary are concerned, it’s not difficult by any means. (I’d say if you can handle Cicero, you can handle Seneca.) Even the metre is pretty straightforward, although you’re going to be exposed to several metrical schemes that may be new to you, but still easy to pick up. I think the biggest hurdle will be remaining alert to the allusive nature of the poetry, but then again if you’ve read the Aeneid or the Metamorpheses, you’ve probably already experienced this. There is a version of Thyestes that contains an excellent commentary by Richard J. Tarrant that’s relatively inexpensive. Here’s a link:
https://www.abebooks.com/servlet/SearchResults?sts=t&cm_sp=SearchF-_-home-_-Results&kn=&an=Tarrant&tn=Thyestes&isbn=
There’s a very good section on metre as well as “the great performance debate”. As you proceed through the play, you’ll discover it’s just the “tip of the iceberg”. You’ll learn some mythology, you’ll be exposed to Stoic philosophy, you’ll learn about the metatheatrical aspects of the play, you’ll learn a bit about Rome during the reign of Nero and finally, you’ll learn to appreciate just how unique Seneca is and how relevant his legacy.