Poor old Mr Musgrove attending the assizes at Taunton would probably only be away for a week at most ;-)

The assizes were established by Magna Carta (1215). They were the local sessions of the High Court of Justice which normally sat in London. To ensure justice was of a local and relatively swift nature the country was divided into Circuits and judges of the High Court would attend each county dispensing justice once or twice a year.

Here is a page from Cary's Itinerary of 1813 which lists the different circuits and shows that Taunton was part of the Western Circuit.

Its assizes were held during Lent , just after the Hilary term. The legal year was divided into four terms: Michaelmas,Hilary,Easter and Trinity. Because the hearings in the country outside London could not clash with the London sessions of the High Court-the same judges were require to attend- it became the custom to hold county assizes in Lent or the Summer (July and August) when the High Court was not sitting in London :

The first attempt to fix specific periods of the year for the holding of sessions by the justices of assize was that made in 1285 by the statute of Westminster II, c.30. Since it was part of a wider rearrangement of the assize system which envisaged the justices of the central courts playing a significant role in the assize circuits the dates chosen for the most part fell within the vacations of those courts; between the quindene of St John the Baptist (8 July) and the Gules of August (1 August), immediately after the end of Trinity; between the feast of the Exaltation of the Cross (14 September) and the octaves of Michaelmas (6 October), immediately preceding Michaelmas term; and between the feast of the Epiphany (6 January) and the feast of the Purification (2 February), overlapping with the start of Hilary term. When the system was recast in 1293 with permanent full-time justices these term limitations were removed but a further reorganization in 1303 brought back the previous assize terms. From the fourteenth century onwards it came to be the norm for assize sessions (whose justices also acted as justices of gaol delivery and nisi prius) to be held in most counties only twice each year. By the sixteenth century these sessions had come to take place during the Lent vacation (after, rather than before, Hilary term) and in the Summer vacation (in July and early August).
See: A Handbook of Dates for Students of British History by C R Cheney and Michael Jones

JA of course, knew of the Taunton assizes from family experience of them. Her aunt Mrs Leigh Perrot went on trial and was acquitted at the Taunton Assizes in 1800 . Because the assizes were only held once a year at Taunton she had had to stay for some months prior to the trial on remand in the home of the gaol keepers house at Ilchester Jail-from August 1799-29th March 1800, in fact.

They were a great opportunity for soliclising and ceremony in addittion to the opportunity to attend important trials for theft and murder etc.

Louis Simond a rather puritanical American who visited England,Scotland and Wales from Christmas Eve 1809 when he landed at Falmouth till the end of September 1811 when he left for New york from Liverpool, wrote an account of his jurney and it ws published anoymously in 1815.

He recorded fabulous details of everyday life in his journal which otherwise would be lost to us and I am very grateful to him for all his detailed observations. And we will be meeting him again soon ;-0

This is what he had to say about the Assizes he attended in York on the 11th March 1811 ( Lent ):

On Sunday the judges, just arrived for the assizes,came to church en grand costume with their huge powdered wigs and black robes; but all three smartness was lost on us who had just seen the Scotch judges dressed in white and pink satin. The mayor and corporation swelled the train and in the rear footmen in white liveries and large nosegays at the button-hole ; the whole town was in motion. The assizes in a country town are an event...

Fanny Burney in Camilla wrote of assizes as follows:

The unfitting, however customary, occasion of this speedy repetition of public amusement in the town of Northwick, was, that the county assizes were now held there; and the arrival of the judges of the land, to hear causes which kept life or death suspended, was the signal for entertainment to the surrounding neighbourhood: a hardening of human feelings against human crimes and human miseries, at which reflection revolts, however habit may persevere.

The young men, who rode on first, joined the ladies as they entered the town, and told them to drive straight to the ballroom, where the company had assembled, in consequence of a shower of rain which had forced them from the public garden intended for the breakfast.
Book 2 Chapter 4

So, Mrs Musgrove is quite in order to feel alarm at her husband being absent: however enjoying oneself at the county assizes does not really compare to a stint on active in the Baltic Im sure you would agree ;-)